Doc: Pacemaker Explained: Add container default for PCMK_logfile

This default applies if PID 1 is pacemaker_remoted or if
PCMK__ENV_REMOTE_PID1 is "full". The former typically applies only in a
container, and the latter is mainly of interest to developers.

Ref T637

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