Help: CPack Archive Generator: Clarify source vs binary package

- Simplify summary
- Revise introduction:
  - Explain how to package source files, provide configuration sample
  - Clarify source vs binary package
- Clarify meaning of variables
   (Mandatory=ON makes no sense if a default is defined)

Help/cpack_gen/archive.rst

@@ -1,8 +1,8 @@
 CPack Archive Generator
 -----------------------
 
-Archive CPack generator that supports packaging of sources and binaries in
-different formats:
+CPack generator for packaging files into an archive, which can have
+any of the following formats:
 
   - 7Z - 7zip - (.7z)
   - TBZ2 (.tar.bz2)
@@ -12,25 +12,42 @@ different formats:
   - TZST (.tar.zst)
   - ZIP (.zip)
 
+When this generator is called from ``CPackSourceConfig.cmake`` (or through
+the ``package_source`` target), then the generated archive will contain all
+files in the project directory, except those specified in
+:variable:`CPACK_SOURCE_IGNORE_FILES`.  The following is one example of
+packaging all source files of a project:
+
+.. code-block:: cmake
+
+  set(CPACK_SOURCE_GENERATOR "TGZ")
+  set(CPACK_SOURCE_IGNORE_FILES
+    \\.git/
+    build/
+    ".*~$"
+  )
+  set(CPACK_VERBATIM_VARIABLES YES)
+  include(CPack)
+
+When this is generator is called from ``CPackConfig.cmake`` (or through the
+``package`` target), then the generated archive will contain all files
+that have been installed via CMake's :command:`install` command (and the
+deprecated commands :command:`install_files`, :command:`install_programs`,
+and :command:`install_targets`).
+
 Variables specific to CPack Archive generator
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 .. variable:: CPACK_ARCHIVE_FILE_NAME
               CPACK_ARCHIVE_<component>_FILE_NAME
 
- Package file name without extension which is added automatically depending
- on the archive format.
-
- * Mandatory : YES
- * Default   : ``<CPACK_PACKAGE_FILE_NAME>[-<component>].<extension>`` with
-               spaces replaced by '-'
+  Package file name without extension. The extension is determined from the
+  archive format (see list above) and automatically appended to the file name.
+  The default is ``<CPACK_PACKAGE_FILE_NAME>[-<component>]``, with spaces
+  replaced by '-'.
 
 .. variable:: CPACK_ARCHIVE_COMPONENT_INSTALL
 
- Enable component packaging for CPackArchive
-
- * Mandatory : NO
- * Default   : OFF
-
- If enabled (ON) multiple packages are generated. By default a single package
- containing files of all components is generated.
+  Enable component packaging. If enabled (ON), then the archive generator
+  creates  multiple packages. The default is OFF, which means that a single
+  package containing files of all components is generated.