Clear --help=language and --help=common interaction.

	PR c/93886
	PR c/93887
	* doc/invoke.texi: Clarify --help=language and --help=common
	interaction.

gcc/ChangeLog

+2020-03-04  Martin Liska  <mliska@suse.cz>
+
+	PR c/93886
+	PR c/93887
+	* doc/invoke.texi: Clarify --help=language and --help=common
+	interaction.
+
 2020-03-04  Jakub Jelinek  <jakub@redhat.com>
 
 	PR tree-optimization/94001

gcc/doc/invoke.texi

@@ -1665,7 +1665,8 @@ option.
 @item @var{language}
 Display the options supported for @var{language}, where
 @var{language} is the name of one of the languages supported in this
-version of GCC@.
+version of GCC@.  If an option is supported by all languages, one needs
+to select @samp{common} class.
 
 @item @samp{common}
 Display the options that are common to all languages.