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