doc: Note that some warnings depend on optimizations (PR 92757)

	PR driver/92757
	* doc/invoke.texi (Warning Options): Add caveat about some warnings
	depending on optimization settings.

gcc/ChangeLog

+2020-01-13  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR driver/92757
+	* doc/invoke.texi (Warning Options): Add caveat about some warnings
+	depending on optimization settings.
+
 2020-01-13  Jakub Jelinek  <jakub@redhat.com>
 
 	PR tree-optimization/90838

gcc/doc/invoke.texi

@@ -4575,6 +4575,11 @@ are being produced.  This allows the use of new @option{-Wno-} options
 with old compilers, but if something goes wrong, the compiler
 warns that an unrecognized option is present.
 
+The effectiveness of some warnings depends on optimizations also being
+enabled. For example @option{-Wsuggest-final-types} is more effective
+with link-time optimization and @option{-Wmaybe-uninitialized} will not
+warn at all unless optimization is enabled.
+
 @table @gcctabopt
 @item -Wpedantic
 @itemx -pedantic