Help: Improve DEBUG_CONFIGURATIONS documentation and usage

This adds a short example demonstrating how to set this property.
Additionally, a paragraph is added to the target_link_libraries() and
its debug/optimized keywords to mention that imported targets with
IMPORTED_CONFIGURATIONS property may provide better alternative for
linking.

Help/command/target_link_libraries.rst

@@ -127,11 +127,15 @@ Each ``<item>`` may be:
   in the :prop_gbl:`DEBUG_CONFIGURATIONS` global property if it is set).
   The ``optimized`` keyword corresponds to all other configurations.  The
   ``general`` keyword corresponds to all configurations, and is purely
-  optional.  Higher granularity may be achieved for per-configuration
-  rules by creating and linking to
-  :ref:`IMPORTED library targets <Imported Targets>`.
-  These keywords are interpreted immediately by this command and therefore
-  have no special meaning when produced by a generator expression.
+  optional.  These keywords are interpreted immediately by this command and
+  therefore have no special meaning when produced by a generator expression.
+
+  Alternatively, generator expressions like :genex:`$<CONFIG>` provide finer
+  per-configuration linking of ``<item>``.  For a more structured approach,
+  higher granularity can be achieved by creating and linking to
+  :ref:`IMPORTED library targets <Imported Targets>` with the
+  :prop_tgt:`IMPORTED_CONFIGURATIONS` property set, particularly in find
+  modules.
 
 Items containing ``::``, such as ``Foo::Bar``, are assumed to be
 :ref:`IMPORTED <Imported Targets>` or :ref:`ALIAS <Alias Targets>` library

Help/prop_gbl/DEBUG_CONFIGURATIONS.rst

@@ -1,13 +1,34 @@
 DEBUG_CONFIGURATIONS
 --------------------
 
-Specify which configurations are for debugging.
+This property specifies which :ref:`Build Configurations` are for debugging.
 
-The value must be a semi-colon separated list of configuration names.
+The value must be a :ref:`semicolon-separated list <CMake Language Lists>` of
+build configuration names.
 Currently this property is used only by the :command:`target_link_libraries`
 command.  Additional uses may be defined in the future.
 
 This property must be set at the top level of the project and before
 the first :command:`target_link_libraries` command invocation.  If any entry in
-the list does not match a valid configuration for the project the
+the list does not match a valid configuration for the project, the
 behavior is undefined.
+
+By default, this property is **not set**.
+
+Examples
+^^^^^^^^
+
+The following example adds a custom configuration to non-optimized debug
+configurations while preserving any existing ones.  If the project uses the
+default ``Debug`` configuration, it should be included as well.
+
+.. code-block:: cmake
+
+  set_property(GLOBAL APPEND PROPERTY DEBUG_CONFIGURATIONS Debug CustomBuild)
+
+See Also
+^^^^^^^^
+
+* The :prop_tgt:`MAP_IMPORTED_CONFIG_<CONFIG>` target property, which maps build
+  configurations when linking to :ref:`Imported Targets` that have the
+  :prop_tgt:`IMPORTED_CONFIGURATIONS` property set.