doc: Correct the timeline for OAuth's shutdown_cb

doc: Correct the timeline for OAuth's shutdown_cb

During original feature development, the OAuth validator shutdown
callback was invoked via before_shmem_exit(). That was changed to use a
reset callback before commit, but I forgot to update the documentation
for validator developers.

Correct this and backport to 18, where OAuth was introduced. The
callback is invoked whenever the server is "finished" with token
validation. (We make no stronger guarantees here, in the hopes that this
API might successfully navigate future multifactor authentication
support and/or changes to the server threading model.)

Reported-by: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAN4CZFOuMb_gnLvCwRdMybg_k8WRNJTjcij%2BPoQkuQHDUzxGWg%40mail.gmail.com
Backpatch-through: 18