BUG: COMMENT was missing from docs.

Source/cmAddCustomCommandCommand.h

@@ -102,10 +102,11 @@ public:
       "  POST_BUILD - run after the target has been built\n"
       "Note that the PRE_BUILD option is only supported on Visual "
       "Studio 7 or later. For all other generators PRE_BUILD "
-      "will be treated as PRE_LINK. "
+      "will be treated as PRE_LINK.\n"
       "If WORKING_DIRECTORY is specified the command will be executed "
-      "in the directory given."
-      "\n"
+      "in the directory given. "
+      "If COMMENT is set, the value will be displayed as a "
+      "message before the commands are executed at build time. "
       "If APPEND is specified the COMMAND and DEPENDS option values "
       "are appended to the custom command for the first output specified. "
       "There must have already been a previous call to this command with "