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>