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.extrawarn b/scripts/Makefile.extrawarn
index d226c5f..53eb7e0 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -8,14 +8,19 @@
 
 KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
 
+# backward compatibility
+KBUILD_EXTRA_WARN ?= $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)
+
 ifeq ("$(origin W)", "command line")
-  export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)
+  KBUILD_EXTRA_WARN := $(W)
 endif
 
+export KBUILD_EXTRA_WARN
+
 #
 # W=1 - warnings which may be relevant and do not occur too often
 #
-ifneq ($(findstring 1, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
 
 KBUILD_CFLAGS += -Wextra -Wunused -Wno-unused-parameter
 KBUILD_CFLAGS += -Wmissing-declarations
@@ -48,7 +53,7 @@
 #
 # W=2 - warnings which occur quite often but may still be relevant
 #
-ifneq ($(findstring 2, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+ifneq ($(findstring 2, $(KBUILD_EXTRA_WARN)),)
 
 KBUILD_CFLAGS += -Wcast-align
 KBUILD_CFLAGS += -Wdisabled-optimization
@@ -65,7 +70,7 @@
 #
 # W=3 - more obscure warnings, can most likely be ignored
 #
-ifneq ($(findstring 3, $(KBUILD_ENABLE_EXTRA_GCC_CHECKS)),)
+ifneq ($(findstring 3, $(KBUILD_EXTRA_WARN)),)
 
 KBUILD_CFLAGS += -Wbad-function-cast
 KBUILD_CFLAGS += -Wcast-qual