load_cache: Improve explanation of command signatures

This extends the descriptions of command signatures and options.
Additional examples are listed at the bottom for better understanding
and overview of this command.

Help/command/load_cache.rst

@@ -1,26 +1,96 @@
 load_cache
 ----------
 
-Load in the values from another project's CMake cache.
+Load in the values from another project's ``CMakeCache.txt`` cache file.  This
+is useful for projects that depend on another project built in a separate
+directory tree.
+
+This command has two signatures.  The recommended signature is:
+
+.. signature::
+  load_cache(<build-dir> READ_WITH_PREFIX <prefix> <entry>...)
+  :target: READ_WITH_PREFIX
+
+  Loads the cache file from the specified ``<build-dir>`` build directory and
+  retrieves the listed cache entries.  The retrieved values are stored in local
+  variables, with their names prefixed by the provided ``<prefix>``.  This only
+  reads the cache values; it does not create or modify entries in the local
+  project's cache.
+
+  ``READ_WITH_PREFIX <prefix>``
+    For each cache ``<entry>``, a local variable is created using the specified
+    ``<prefix>`` followed by the entry name.
+
+The following signature of this command is strongly discouraged, but it is
+provided for backward compatibility:
+
+.. signature::
+  load_cache(<build-dir> [EXCLUDE <entry>...] [INCLUDE_INTERNALS <entry>...])
+  :target: raw
+
+  This form loads the cache file from the specified ``<build-dir>`` build
+  directory and imports all its non-internal cache entries into the local
+  project's cache as internal cache variables.  By default, only non-internal
+  entries are imported, unless the ``INCLUDE_INTERNALS`` option is used.
+
+  The options are:
+
+  ``EXCLUDE <entry>...``
+    This option can be used to exclude a given list of non-internal cache
+    entries when importing values.
+  ``INCLUDE_INTERNALS <entry>...``
+    This option can be used to provide a list of internal cache entries to
+    include in addition to the non-internal cache entries.
+
+Examples
+^^^^^^^^
+
+Reading specific cache variables from another project and storing them as local
+variables:
 
 .. code-block:: cmake
 
-  load_cache(pathToBuildDirectory READ_WITH_PREFIX prefix entry1...)
+  load_cache(
+    path/to/other-project/build-dir
+    READ_WITH_PREFIX prefix_
+    OTHER_PROJECT_CACHE_VAR_1
+    OTHER_PROJECT_CACHE_VAR_2
+  )
 
-Reads the cache and store the requested entries in variables with their
-name prefixed with the given prefix.  This only reads the values, and
-does not create entries in the local project's cache.
+  message(STATUS "${prefix_OTHER_PROJECT_CACHE_VAR_1")
+  message(STATUS "${prefix_OTHER_PROJECT_CACHE_VAR_2")
+  # Outputs:
+  # -- some-value...
+  # -- another-value...
+
+Reading all non-internal cache entries from another project and storing them as
+internal cache variables using the obsolete signature:
 
 .. code-block:: cmake
 
-  load_cache(pathToBuildDirectory [EXCLUDE entry1...]
-             [INCLUDE_INTERNALS entry1...])
+  load_cache(path/to/other-project/build-dir)
 
-Loads in the values from another cache and store them in the local
-project's cache as internal entries.  This is useful for a project
-that depends on another project built in a different tree.  ``EXCLUDE``
-option can be used to provide a list of entries to be excluded.
-``INCLUDE_INTERNALS`` can be used to provide a list of internal entries to
-be included.  Normally, no internal entries are brought in.  Use of
-this form of the command is strongly discouraged, but it is provided
-for backward compatibility.
+  message(STATUS "${OTHER_PROJECT_CACHE_VAR_1")
+  message(STATUS "${OTHER_PROJECT_CACHE_VAR_2")
+  # Outputs:
+  # -- some-value...
+  # -- another-value...
+
+Excluding specific non-internal cache entries and including internal ones using
+the obsolete signature:
+
+.. code-block:: cmake
+
+  load_cache(
+    path/to/other-project/build-dir
+    EXCLUDE OTHER_PROJECT_CACHE_VAR_2
+    INCLUDE_INTERNALS OTHER_PROJECT_INTERNAL_CACHE_VAR
+  )
+
+  message(STATUS "${OTHER_PROJECT_CACHE_VAR_1")
+  message(STATUS "${OTHER_PROJECT_CACHE_VAR_2")
+  message(STATUS "${OTHER_PROJECT_INTERNAL_CACHE_VAR}")
+  # Outputs:
+  # -- some-value...
+  # --
+  # -- some-internal-value...