Mention standby.signal in FATALs for checkpoint record missing at recovery

Mention standby.signal in FATALs for checkpoint record missing at recovery

When beginning recovery from a base backup by reading a backup_label
file, it may be possible that no checkpoint record is available
depending on the method used when the case backup was taken, which would
prevent recovery from beginning.  In this case, the FATAL messages
issued, initially added by c900c15269f0f, mentioned recovery.signal as
an option to do recovery but not standby.signal.  Let's add it as an
available option, for clarity.

Per suggestion from Bowen Shi, extracted from a larger patch by me.

Author: Michael Paquier
Discussion: https://postgr.es/m/CAM_vCudkSjr7NsNKSdjwtfAm9dbzepY6beZ5DP177POKy8=2aw@mail.gmail.com