docs: Improve plugin documentation (zyedidia#3240)

* docs: Improve documentation

`commands.md`
  - documented the `reopen` command
  - improved the documentation of the `reload` command

`plugins.md`
  - added direct links to relevant external documentation pages
  - rewrote some sections
  - documented missing functions/callbacks
  - added a note about installing/managing plugins

* Omit number of default plugins

Co-authored-by: Jöran Karl <[email protected]>

---------

Co-authored-by: Jöran Karl <[email protected]>

Author: glupi-borna

runtime/help/commands.md

@@ -107,14 +107,19 @@ quotes here but these are not necessary when entering the command in micro.
 
 * `plugin available`: show available plugins that can be installed.
 
-* `reload`: reloads all runtime files.
+* `reload`: reloads all runtime files (settings, keybindings, syntax files,
+   colorschemes, plugins). All plugins will be unloaded by running their
+   `deinit()` function (if it exists), and then loaded again by calling the
+   `preinit()`, `init()` and `postinit()` functions (if they exist).
 
 * `cd 'path'`: Change the working directory to the given `path`.
 
 * `pwd`: Print the current working directory.
 
 * `open 'filename'`: Open a file in the current buffer.
 
+* `reopen`: Reopens the current file from disk.
+
 * `reset 'option'`: resets the given option to its default value
 
 * `retab`: Replaces all leading tabs with spaces or leading spaces with tabs