Update glossary for package and project. Refine online docs.

Author: mpilgrem

doc/glossary.md

@@ -36,9 +36,11 @@ The following terms are used in Stack's documentation.
 |Make               |A [build automation tool](https://www.gnu.org/software/make/).|
 |MSYS2              |The [MSYS2](https://www.msys2.org/) software distribution and building platform for Windows.|
 |Nix                |A purely functional [package manager](https://nixos.org/), available for Linux and macOS.|
+|package            |A Haskell package is an organised collection of Haskell code and related files. It is described by a Cabal file or a `package.yaml` file, which is itself part of the package.|
 |`package.yaml`     |A file that describes a package in the Hpack format.      |
 |Pantry             |A library for content-addressable Haskell package management, provided by the [`pantry` package](https://hackage.haskell.org/package/pantry). A dependency of Stack.|
 |PATH               |The `PATH` environment variable, specifying a list of directories searched for executable files.|
+|project            |A Stack project is a local directory that contains a project-level configuration file (`stack.yaml`). A project may relate to more than one local package.|
 |PVP                |The Haskell [Package Versioning Policy](https://pvp.haskell.org/), which tells developers of libraries how to set their version numbers.|
 |REPL               |An interactive (run-eval-print loop) programming environment.|
 |resolver           |A synonym for snapshot.                                   |

doc/yaml_configuration.md

@@ -5,6 +5,18 @@
 Stack is configured by the content of YAML files. Some Stack operations can also
 be customised by the use of scripts.
 
+!!! note
+
+    A Haskell package is an organised collection of Haskell code and related
+    files. It is described by a Cabal file or a `package.yaml` file (which can
+    be used to generate a Cabal file). The package description is itself part of
+    the package. Its file is located in the root directory of a local package.
+
+    A Stack project is a local directory that contains a Stack project-level
+    configuration file (`stack.yaml`). A project may relate to more than one
+    local package. A single-package project's directory will usually also be the
+    package's root directory.
+
 ## YAML configuration
 
 Stack's YAML configuration options break down into
@@ -69,12 +81,6 @@ Cabal file (named `<package_name>.cabal`), see the
 Project-specific configuration options are valid only in a project-level
 configuration file (`stack.yaml`).
 
-!!! note
-
-    We define **project** to mean a directory that contains a `stack.yaml`
-    file, which specifies how to build a set of packages. We define **package** to
-    be a package with a Cabal file or an Hpack `package.yaml` file.
-
 In your project-specific options, you specify both **which local packages** to
 build and **which dependencies to use** when building these packages. Unlike the
 user's local packages, these dependencies aren't built by default. They only get
@@ -269,11 +275,14 @@ drop-packages:
 
 !!! info
 
-    In older snapshots, it used to be handy to drop Cabal for reasons listed in
-    Stackage issue
-    [#4425](https://github.com/commercialhaskell/stackage/issues/4425). However,
-    since around February 2020 (LTS-14.27 and nightly-2020-02-08), Cabal is not
-    directly included in snapshots, so dropping Cabal no longer has any effect.
+    Stackage snapshots LTS Haskell 14.27 (GHC 8.6.5) and earlier, and Nightly
+    2022-02-08 (GHC 8.8.2) and earlier, included directly the `Cabal` package.
+    Later snapshots do not include directly that package (which is a GHC boot
+    package).
+
+    For the older Stackage snapshots, it could be handy to drop the
+    snapshot-specified `Cabal` package, to avoid building that version of the
+    package. For the later snapshots, there is no package version to drop.
 
 ### user-message