Remove an outdated comment as requested by Bruce Evans in a private email to

Alexander Best (arundel@).

For clang, -fdiagnostics-show-option is enabled by default, but for gcc it
isn't. This option will report which -W* flag was responsible for triggering
a certain warning. This will bring gcc warnings closer to the ones clang emits
and might also help developers track down tinderbox failures a bit quicker.

Submitted by:	arundel

sys/conf/kern.mk

@@ -1,15 +1,12 @@
 # $FreeBSD$
 
 #
-# Warning flags for compiling the kernel and components of the kernel.
+# Warning flags for compiling the kernel and components of the kernel:
 #
-# Note that the newly added -Wcast-qual is responsible for generating
-# most of the remaining warnings.  Warnings introduced with -Wall will
-# also pop up, but are easier to fix.
 CWARNFLAGS?=	-Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual \
 		-Wundef -Wno-pointer-sign -fformat-extensions \
-		-Wmissing-include-dirs
+		-Wmissing-include-dirs -fdiagnostics-show-option
 #
 # The following flags are next up for working on:
 #	-Wextra