Help: Describe options of command project in a definition list.

Also add section headers;
explain default setting of optional argument variables.

Help/command/project.rst

@@ -1,7 +1,10 @@
 project
 -------
 
-Sets project details such as name, version, etc. and enables languages.
+Set the name of the project.
+
+Synopsis
+^^^^^^^^
 
 .. code-block:: cmake
 
@@ -12,65 +15,83 @@ Sets project details such as name, version, etc. and enables languages.
          [HOMEPAGE_URL <url-string>]
          [LANGUAGES <language-name>...])
 
-Sets the name of the project and stores the name in the
-:variable:`PROJECT_NAME` variable.  Additionally this sets variables
+Sets the name of the project, and stores it in the variable
+:variable:`PROJECT_NAME`.  Also sets the variables
 
 * :variable:`PROJECT_SOURCE_DIR`,
   :variable:`<PROJECT-NAME>_SOURCE_DIR`
 * :variable:`PROJECT_BINARY_DIR`,
   :variable:`<PROJECT-NAME>_BINARY_DIR`
 
-If ``VERSION`` is specified, given components must be non-negative integers.
-If ``VERSION`` is not specified, the default version is the empty string.
-The ``VERSION`` option may not be used unless policy :policy:`CMP0048` is
-set to ``NEW``.
-
-The :command:`project()` command stores the version number and its components
-in variables
-
-* :variable:`PROJECT_VERSION`,
-  :variable:`<PROJECT-NAME>_VERSION`
-* :variable:`PROJECT_VERSION_MAJOR`,
-  :variable:`<PROJECT-NAME>_VERSION_MAJOR`
-* :variable:`PROJECT_VERSION_MINOR`,
-  :variable:`<PROJECT-NAME>_VERSION_MINOR`
-* :variable:`PROJECT_VERSION_PATCH`,
-  :variable:`<PROJECT-NAME>_VERSION_PATCH`
-* :variable:`PROJECT_VERSION_TWEAK`,
-  :variable:`<PROJECT-NAME>_VERSION_TWEAK`
-
-Variables corresponding to unspecified versions are set to the empty string
-(if policy :policy:`CMP0048` is set to ``NEW``).
-
-If the optional ``DESCRIPTION`` is given, then :variable:`PROJECT_DESCRIPTION`
-and :variable:`<PROJECT-NAME>_DESCRIPTION` will be set to its argument.
-These variables will be cleared if ``DESCRIPTION`` is not given.
-The description is expected to be a relatively short string, usually no more
-than a few words.
-
-The optional ``HOMEPAGE_URL`` sets the analogous variables
-:variable:`PROJECT_HOMEPAGE_URL` and :variable:`<PROJECT-NAME>_HOMEPAGE_URL`.
-When this option is given, the URL provided should be the canonical home for
-the project.
-These variables will be cleared if ``HOMEPAGE_URL`` is not given.
-
-Note that the description and homepage URL may be used as defaults for
-things like packaging meta-data, documentation, etc.
-
-Optionally you can specify which languages your project supports.
-Example languages include ``C``, ``CXX`` (i.e.  C++), ``CUDA``,
-``Fortran``, and ``ASM``.
-By default ``C`` and ``CXX`` are enabled if no language options are
-given.  Specify language ``NONE``, or use the ``LANGUAGES`` keyword
-and list no languages, to skip enabling any languages.
-
-If enabling ``ASM``, list it last so that CMake can check whether
-compilers for other languages like ``C`` work for assembly too.
-
-If a variable exists called :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE`,
+Further variables are set by the optional arguments described in the following.
+If any of these arguments is not used, then the corresponding variables are
+set to the empty string.
+
+If the variable :variable:`CMAKE_PROJECT_<PROJECT-NAME>_INCLUDE` exists,
 the file pointed to by that variable will be included as the last step of the
 project command.
 
+Options
+^^^^^^^
+
+The options are:
+
+``VERSION <version>``
+  Optional; may not be used unless policy :policy:`CMP0048` is
+  set to ``NEW``.
+
+  Takes a ``<version>`` argument composed of non-negative integer components,
+  i.e. ``<major>[.<minor>[.<patch>[.<tweak>]]]``,
+  and sets the variables
+
+  * :variable:`PROJECT_VERSION`,
+    :variable:`<PROJECT-NAME>_VERSION`
+  * :variable:`PROJECT_VERSION_MAJOR`,
+    :variable:`<PROJECT-NAME>_VERSION_MAJOR`
+  * :variable:`PROJECT_VERSION_MINOR`,
+    :variable:`<PROJECT-NAME>_VERSION_MINOR`
+  * :variable:`PROJECT_VERSION_PATCH`,
+    :variable:`<PROJECT-NAME>_VERSION_PATCH`
+  * :variable:`PROJECT_VERSION_TWEAK`,
+    :variable:`<PROJECT-NAME>_VERSION_TWEAK`.
+
+``DESCRIPTION <project-description-string>``
+  Optional.
+  Sets the variables
+
+  * :variable:`PROJECT_DESCRIPTION`, :variable:`<PROJECT-NAME>_DESCRIPTION`
+
+  to ``<project-description-string>``.
+  It is recommended that this description is a relatively short string,
+  usually no more than a few words.
+
+``HOMEPAGE_URL <url-string>``
+  Optional.
+  Sets the variables
+
+  * :variable:`PROJECT_HOMEPAGE_URL`, :variable:`<PROJECT-NAME>_HOMEPAGE_URL`
+
+  to ``<url-string>``, which should be the canonical home URL for the project.
+
+``LANGUAGES <language-name>...``
+  Optional.
+  Can also be specified without ``LANGUAGES`` keyword per the first, short signature.
+
+  Selects which programming languages are needed to build the project.
+  Supported languages include ``C``, ``CXX`` (i.e.  C++), ``CUDA``, ``Fortran``, and ``ASM``.
+  By default ``C`` and ``CXX`` are enabled if no language options are given.
+  Specify language ``NONE``, or use the ``LANGUAGES`` keyword and list no languages,
+  to skip enabling any languages.
+
+  If enabling ``ASM``, list it last so that CMake can check whether
+  compilers for other languages like ``C`` work for assembly too.
+
+The variables set through the ``VERSION``, ``DESCRIPTION`` and ``HOMEPAGE_URL``
+options are intended for use as default values in package metadata and documentation.
+
+Usage
+^^^^^
+
 The top-level ``CMakeLists.txt`` file for a project must contain a
 literal, direct call to the :command:`project` command; loading one
 through the :command:`include` command is not sufficient.  If no such