kbuild: add comment for V=2 mode

Complete the comments for valid values of KBUILD_VERBOSE,
specifically for KBUILD_VERBOSE=2.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -68,6 +68,7 @@ unexport GREP_OPTIONS
 #
 # If KBUILD_VERBOSE equals 0 then the above command will be hidden.
 # If KBUILD_VERBOSE equals 1 then the above command is displayed.
+# If KBUILD_VERBOSE equals 2 then give the reason why each target is rebuilt.
 #
 # To put more focus on warnings, be less verbose as default
 # Use 'make V=1' to see the full commands