Refactor: libpacemaker: Clarify pointer arguments in pcmk_status.c

Make only_node and only_rsc arguments const. Add doxygen blocks. Change
"st" argument to "stonith".

This is not comprehensive. We're updating pcmkstatus() because we're
about to add a new argument, and pcmkoutput_cluster_status() because
it shares most of its arguments with pcmk__status().

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