add_key.2, request_key.2, keyrings.7: Update Linux documentation pointers

add_key.2, request_key.2, keyrings.7: Update Linux documentation pointers

In 4.13 the release cycle, key management documentation has been
reformatted to ReStructured text and moved to a separate
"keys" directory.

Relevant kernel commits: v4.13-rc1~34^2~27, v4.13-rc1~34^2~25

* man2/add_key.2 (.SH SEE ALSO): Amend pointers to
Documentation/security/keys.txt and Documentation/keys-request-key.txt
with changes introduced in Linux 4.13 (Documentation/keys/core.rst and
Documentation/keys/request-key.rst).
* man2/request_key.2 (.SH SEE ALSO): Likewise.
* man7/keyrings.7 (.SH SEE ALSO): Likewise.

Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>