Documentation/ABI: document the non-ABI status of Kconfig and symbols

Discussion at Kernel Summit made it clear that the presence or absence of
specific Kconfig symbols are not considered ABI, and that no userspace (or
bootloader, etc) should rely on them.

In addition, kernel-internal symbols are well established as non-ABI, per
Documentation/stable_api_nonsense.txt.

Document both of these in Documentation/ABI/README, in a new section for
notable bits of non-ABI.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Cc: Rob Landley <rob@landley.net>
Cc: Tao Ma <boyu.mt@taobao.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Cc: Richard Weinberger <richard.weinberger@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/README

@@ -72,3 +72,16 @@ kernel tree without going through the obsolete state first.
 
 It's up to the developer to place their interfaces in the category they
 wish for it to start out in.
+
+
+Notable bits of non-ABI, which should not under any circumstances be considered
+stable:
+
+- Kconfig.  Userspace should not rely on the presence or absence of any
+  particular Kconfig symbol, in /proc/config.gz, in the copy of .config
+  commonly installed to /boot, or in any invocation of the kernel build
+  process.
+
+- Kernel-internal symbols.  Do not rely on the presence, absence, location, or
+  type of any kernel symbol, either in System.map files or the kernel binary
+  itself.  See Documentation/stable_api_nonsense.txt.