Doc: sysconfig: Mark PCMK_cib_timeout as deprecated

It's undocumented and the string constant is internal-only, so this only
requires COMPAT comments.

See discussion at
https://github.com/ClusterLabs/pacemaker/pull/3245#discussion_r1372317358.

Ref T637

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>