crypto: af_alg - Document the deprecation of AF_ALG

AF_ALG is almost completely unnecessary, and it exposes a massive attack
surface that hasn't been standing up to modern vulnerability discovery
tools.  The latest one even has its own website, providing a small
Python script that reliably roots most Linux distros: https://copy.fail/

This isn't sustainable, especially as LLMs have accelerated the rate the
vulnerabilities are coming in.  The effort that is being put into this
thing is vastly disproportional to the few programs that actually use
it, and those programs would be better served by userspace code anyway.

These issues have been noted in many mailing list discussions already.
But until now they haven't been reflected in the documentation or
kconfig menu itself, and the vulnerabilities are still coming in.

Let's go ahead and document the deprecation.

This isn't intended to change anything overnight.  After all, most Linux
distros won't be able to disable the kconfig options quite yet, mainly
because of iwd.  But this should create a bit more impetus for these
userspace programs to be fixed, and the documentation update should also
help prevent more users from appearing.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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