[MINOR][DOCS] Minor doc fixes related with doc build and uses script dir in SQL doc gen script

## What changes were proposed in this pull request?

This PR proposes both:

- Add information about Javadoc, SQL docs and few more information in `docs/README.md` and a comment in `docs/_plugins/copy_api_dirs.rb` related with Javadoc.

- Adds some commands so that the script always runs the SQL docs build under `./sql` directory (for directly running `./sql/create-docs.sh` in the root directory).

## How was this patch tested?

Manual tests with `jekyll build` and `./sql/create-docs.sh` in the root directory.

Author: hyukjinkwon <[email protected]>

Closes apache#19019 from HyukjinKwon/minor-doc-build.

Author: HyukjinKwon

docs/README.md

@@ -9,19 +9,22 @@ documentation yourself. Why build it yourself? So that you have the docs that co
 whichever version of Spark you currently have checked out of revision control.
 
 ## Prerequisites
-The Spark documentation build uses a number of tools to build HTML docs and API docs in Scala,
-Python and R.
+
+The Spark documentation build uses a number of tools to build HTML docs and API docs in Scala, Java,
+Python, R and SQL.
 
 You need to have [Ruby](https://www.ruby-lang.org/en/documentation/installation/) and
 [Python](https://docs.python.org/2/using/unix.html#getting-and-installing-the-latest-version-of-python)
 installed. Also install the following libraries:
+
 ```sh
-    $ sudo gem install jekyll jekyll-redirect-from pygments.rb
-    $ sudo pip install Pygments
-    # Following is needed only for generating API docs
-    $ sudo pip install sphinx pypandoc mkdocs
-    $ sudo Rscript -e 'install.packages(c("knitr", "devtools", "roxygen2", "testthat", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
+$ sudo gem install jekyll jekyll-redirect-from pygments.rb
+$ sudo pip install Pygments
+# Following is needed only for generating API docs
+$ sudo pip install sphinx pypandoc mkdocs
+$ sudo Rscript -e 'install.packages(c("knitr", "devtools", "roxygen2", "testthat", "rmarkdown"), repos="http://cran.stat.ucla.edu/")'
 ```
+
 (Note: If you are on a system with both Ruby 1.9 and Ruby 2.0 you may need to replace gem with gem2.0)
 
 ## Generating the Documentation HTML
@@ -32,42 +35,49 @@ the source code and be captured by revision control (currently git). This way th
 includes the version of the documentation that is relevant regardless of which version or release
 you have checked out or downloaded.
 
-In this directory you will find textfiles formatted using Markdown, with an ".md" suffix. You can
-read those text files directly if you want. Start with index.md.
+In this directory you will find text files formatted using Markdown, with an ".md" suffix. You can
+read those text files directly if you want. Start with `index.md`.
 
 Execute `jekyll build` from the `docs/` directory to compile the site. Compiling the site with
-Jekyll will create a directory called `_site` containing index.html as well as the rest of the
+Jekyll will create a directory called `_site` containing `index.html` as well as the rest of the
 compiled files.
 
-    $ cd docs
-    $ jekyll build
+```sh
+$ cd docs
+$ jekyll build
+```
 
 You can modify the default Jekyll build as follows:
+
 ```sh
-    # Skip generating API docs (which takes a while)
-    $ SKIP_API=1 jekyll build
-    
-    # Serve content locally on port 4000
-    $ jekyll serve --watch
-    
-    # Build the site with extra features used on the live page
-    $ PRODUCTION=1 jekyll build
+# Skip generating API docs (which takes a while)
+$ SKIP_API=1 jekyll build
+
+# Serve content locally on port 4000
+$ jekyll serve --watch
+
+# Build the site with extra features used on the live page
+$ PRODUCTION=1 jekyll build
 ```
 
-## API Docs (Scaladoc, Sphinx, roxygen2)
+## API Docs (Scaladoc, Javadoc, Sphinx, roxygen2, MkDocs)
 
-You can build just the Spark scaladoc by running `build/sbt unidoc` from the SPARK_PROJECT_ROOT directory.
+You can build just the Spark scaladoc and javadoc by running `build/sbt unidoc` from the `SPARK_HOME` directory.
 
 Similarly, you can build just the PySpark docs by running `make html` from the
-SPARK_PROJECT_ROOT/python/docs directory. Documentation is only generated for classes that are listed as
-public in `__init__.py`. The SparkR docs can be built by running SPARK_PROJECT_ROOT/R/create-docs.sh.
+`SPARK_HOME/python/docs` directory. Documentation is only generated for classes that are listed as
+public in `__init__.py`. The SparkR docs can be built by running `SPARK_HOME/R/create-docs.sh`, and
+the SQL docs can be built by running `SPARK_HOME/sql/create-docs.sh`
+after [building Spark](https://github.com/apache/spark#building-spark) first.
 
-When you run `jekyll` in the `docs` directory, it will also copy over the scaladoc for the various
+When you run `jekyll build` in the `docs` directory, it will also copy over the scaladoc and javadoc for the various
 Spark subprojects into the `docs` directory (and then also into the `_site` directory). We use a
 jekyll plugin to run `build/sbt unidoc` before building the site so if you haven't run it (recently) it
-may take some time as it generates all of the scaladoc.  The jekyll plugin also generates the
-PySpark docs using [Sphinx](http://sphinx-doc.org/).
+may take some time as it generates all of the scaladoc and javadoc using [Unidoc](https://github.com/sbt/sbt-unidoc).
+The jekyll plugin also generates the PySpark docs using [Sphinx](http://sphinx-doc.org/), SparkR docs
+using [roxygen2](https://cran.r-project.org/web/packages/roxygen2/index.html) and SQL docs
+using [MkDocs](http://www.mkdocs.org/).
 
-NOTE: To skip the step of building and copying over the Scala, Python, R and SQL API docs, run `SKIP_API=1
-jekyll`. In addition, `SKIP_SCALADOC=1`, `SKIP_PYTHONDOC=1`, `SKIP_RDOC=1` and `SKIP_SQLDOC=1` can be used
-to skip a single step of the corresponding language.
+NOTE: To skip the step of building and copying over the Scala, Java, Python, R and SQL API docs, run `SKIP_API=1
+jekyll build`. In addition, `SKIP_SCALADOC=1`, `SKIP_PYTHONDOC=1`, `SKIP_RDOC=1` and `SKIP_SQLDOC=1` can be used
+to skip a single step of the corresponding language. `SKIP_SCALADOC` indicates skipping both the Scala and Java docs.

docs/_plugins/copy_api_dirs.rb

@@ -20,7 +20,7 @@
 
 if not (ENV['SKIP_API'] == '1')
   if not (ENV['SKIP_SCALADOC'] == '1')
-    # Build Scaladoc for Java/Scala
+    # Build Scaladoc for Scala and Javadoc for Java
 
     puts "Moving to project root and building API docs."
     curr_dir = pwd

sql/create-docs.sh

@@ -37,6 +37,8 @@ if ! hash mkdocs 2>/dev/null; then
   pip install mkdocs
 fi
 
+pushd "$FWDIR" > /dev/null
+
 # Now create the markdown file
 rm -fr docs
 mkdir docs
@@ -47,3 +49,5 @@ echo "Generating markdown files for SQL documentation."
 echo "Generating HTML files for SQL documentation."
 mkdocs build --clean
 rm -fr docs
+
+popd