doc/ux.texi: auto_diagnostic_group now does something

As of r266186, auto_diagnostic_group is now user-visible
(via -fdiagnostics-format=json), so update the diagnostic
guidelines accordingly.

gcc/ChangeLog:
	* doc/ux.texi (Group logically-related diagnostics): Move
	discussion of auto_diagnostic_group into this new subsection.
	Give an example of where this grouping is used.

From-SVN: r266187

gcc/ChangeLog

 2018-11-15  David Malcolm  <dmalcolm@redhat.com>
 
+	* doc/ux.texi (Group logically-related diagnostics): Move
+	discussion of auto_diagnostic_group into this new subsection.
+	Give an example of where this grouping is used.
+
+2018-11-15  David Malcolm  <dmalcolm@redhat.com>
+
 	PR other/19165
 	* Makefile.in (OBJS): Move json.o to...
 	(OBJS-libcommon): ...here and add diagnostic-format-json.o.

gcc/doc/ux.texi

@@ -376,12 +376,15 @@ where the @code{double} and @code{int} are colorized to highlight them.
 
 @c %H and %I were added in r248698.
 
+@subsection Group logically-related diagnostics
+
 Use @code{auto_diagnostic_group} when issuing multiple related
 diagnostics (seen in various examples on this page).  This informs the
 diagnostic subsystem that all diagnostics issued within the lifetime
-of the @code{auto_diagnostic_group} are related.  (Currently it doesn't
-do anything with this information, but we may implement that in the
-future).
+of the @code{auto_diagnostic_group} are related.  For example,
+@option{-fdiagnostics-format=json} will treat the first diagnostic
+emitted within the group as a top-level diagnostic, and all subsequent
+diagnostics within the group as its children.
 
 @subsection Quoting
 Text should be quoted by either using the @samp{q} modifier in a directive