kconfig.h: explain IS_MODULE(), IS_ENABLED()

Extend IS_MODULE() and IS_ENABLED comments to explain why one might use
"#if IS_ENABLED(CONFIG_FOO)" instead of "#ifdef CONFIG_FOO".

To wit, "#ifdef CONFIG_FOO" is true only for CONFIG_FOO=y, while
"#if IS_ENABLED(CONFIG_FOO)" is true for both CONFIG_FOO=y and
CONFIG_FOO=m.

This is because "CONFIG_FOO=m" in .config does not result in "CONFIG_FOO"
being defined.  The actual definitions are in autoconf.h, where:

  CONFIG_FOO=y   results in   #define CONFIG_FOO 1
  CONFIG_FOO=m   results in   #define CONFIG_FOO_MODULE 1
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

include/linux/kconfig.h

@@ -51,7 +51,8 @@
 
 /*
  * IS_MODULE(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'm', 0
- * otherwise.
+ * otherwise.  CONFIG_FOO=m results in "#define CONFIG_FOO_MODULE 1" in
+ * autoconf.h.
  */
 #define IS_MODULE(option) __is_defined(option##_MODULE)
 
@@ -66,7 +67,8 @@
 
 /*
  * IS_ENABLED(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y' or 'm',
- * 0 otherwise.
+ * 0 otherwise.  Note that CONFIG_FOO=y results in "#define CONFIG_FOO 1" in
+ * autoconf.h, while CONFIG_FOO=m results in "#define CONFIG_FOO_MODULE 1".
  */
 #define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))