Clearly document behaviour of multiple -g options

This copies the wording from the -O options to clearly state what
happens if more than one -g option is used.

	* doc/invoke.texi (Debugging Options): Explicitly state the semantics
	of using multiple -g options.

From-SVN: r270257

gcc/ChangeLog

+2019-04-10  Jonathan Wakely  <jwakely@redhat.com>
+
+	* doc/invoke.texi (Debugging Options): Explicitly state the semantics
+	of using multiple -g options.
+
 2019-04-10  Martin Liska  <mliska@suse.cz>
 
 	PR gcov-profile/89959

gcc/doc/invoke.texi

@@ -7818,6 +7818,9 @@ Level 3 includes extra information, such as all the macro definitions
 present in the program.  Some debuggers support macro expansion when
 you use @option{-g3}.
 
+If you use multiple @option{-g} options, with or without level numbers,
+the last such option is the one that is effective.
+
 @option{-gdwarf} does not accept a concatenated debug level, to avoid
 confusion with @option{-gdwarf-@var{level}}.
 Instead use an additional @option{-g@var{level}} option to change the