ENH: Added example of sub-group to docs.

Source/cmSourceGroupCommand.h

@@ -68,8 +68,11 @@ public:
       "be placed in this group.  If a file matches multiple groups, the LAST "
       "group that explicitly lists the file will be favored, if any.  If no "
       "group explicitly lists the file, the LAST group whose regular "
-      "expression matches the file will be favored. For backwards "
-      "compatibility, this command is also supports the format:\n"
+      "expression matches the file will be favored.\n"
+      "The name of the group may contain backslashes to specify subgroups:\n"
+      "  SOURCE_GROUP(outer\\\\inner ...)\n"
+      "For backwards compatibility, this command is also supports the "
+      "format:\n"
       "  SOURCE_GROUP(name regex)";
     }