Mention in bgworker docs that db connection needs shmem access

Hi all,

The bgworker documentation does not explicitely mention that a
bgworker using BGWORKER_BACKEND_DATABASE_CONNECTION needs to have
BGWORKER_SHMEM_ACCESS set as well.

Just to mention, a bgworker using only db connection flag and not
shmem flag will fail at atart-up with this error.
background worker "hello signal worker": must attach to shared memory
in order to request a database connection

Please find attached a patch that improves documentation a bit by
mentioning this restriction. This should normally be backpatched to
9.3 stable.

Thanks,
--
Michael