Mention jsonlog in description of logging_collector in GUC table

Mention jsonlog in description of logging_collector in GUC table

logging_collector was only mentioning stderr and csvlog, and forgot
about jsonlog.  Oversight in dc686681e079, that has added support for
jsonlog in log_destination.

While on it, the description in the GUC table is tweaked to be more
consistent with the documentation and postgresql.conf.sample.

Author: Umar Hayat
Reviewed-by: Ashutosh Bapat, Tom Lane
Discussion: https://postgr.es/m/CAD68Dp1K_vBYqBEukHw=1jF7e76t8aszGZTFL2ugi=H7r=a7MA@mail.gmail.com
Backpatch-through: 13