Help: Add link in set_source_files_properties command docs.

Remove claim about unrecognized properties being ignored.  Such
properties can be retrieved later by the user.

Help/command/set_source_files_properties.rst

@@ -10,6 +10,6 @@ Source files can have properties that affect how they are built.
                               [prop2 value2 [...]])
 
 Set properties associated with source files using a key/value paired
-list.  See properties documentation for those known to CMake.
-Unrecognized properties are ignored.  Source file properties are
-visible only to targets added in the same directory (CMakeLists.txt).
+list.  See :ref:`Source File Properties` for the list of properties known
+to CMake.  Source file properties are visible only to targets added
+in the same directory (CMakeLists.txt).