Help:IMPORTED_IMPLIB: Mention IMPORTED_IMPLIB_<CONFIG> variant

This is already done in `IMPORTED_LOCATION` documentation.

Help/prop_tgt/IMPORTED_IMPLIB.rst

@@ -12,4 +12,13 @@ This property may be set:
   the :prop_tgt:`ENABLE_EXPORTS` target property). For frameworks this is the
   location of the ``.tbd`` file symlink just inside the framework folder.
 
+The ``IMPORTED_IMPLIB`` target property may be overridden for a
+given configuration ``<CONFIG>`` by the configuration-specific
+:prop_tgt:`IMPORTED_IMPLIB_<CONFIG>` target property.  Furthermore,
+the :prop_tgt:`MAP_IMPORTED_CONFIG_<CONFIG>` target property may be
+used to map between a project's configurations and those of an imported
+target.  If none of these is set then the name of any other configuration
+listed in the :prop_tgt:`IMPORTED_CONFIGURATIONS` target property may be
+selected and its :prop_tgt:`IMPORTED_IMPLIB_<CONFIG>` value used.
+
 This property is ignored for non-imported targets.