kbuild: rename KBUILD_ENABLE_EXTRA_GCC_CHECKS to KBUILD_EXTRA_WARN

KBUILD_ENABLE_EXTRA_GCC_CHECKS started as a switch to add extra warning
options for GCC, but now it is a historical misnomer since we use it
also for Clang, DTC, and even kernel-doc.

Rename it to more sensible, shorter KBUILD_EXTRA_WARN.

For the backward compatibility, KBUILD_ENABLE_EXTRA_GCC_CHECKS is still
supported (but not advertised in the documentation).

I also fixed up 'make help', and updated the documentation.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 2a21ca8..f72aba6 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -85,7 +85,7 @@
         cmd_force_checksrc = $(CHECK) $(CHECKFLAGS) $(c_flags) $<
 endif
 
-ifneq ($(KBUILD_ENABLE_EXTRA_GCC_CHECKS),)
+ifneq ($(KBUILD_EXTRA_WARN),)
   cmd_checkdoc = $(srctree)/scripts/kernel-doc -none $<
 endif