doc: Document default values for pgoutput options in protocol.sgml.

doc: Document default values for pgoutput options in protocol.sgml.

The pgoutput plugin options are described in the logical streaming
replication protocol documentation, but their default values were
previously not mentioned. This made it less convenient for users,
for example, when specifying those options to use pg_recvlogical
with pgoutput plugin.

This commit adds the explanations of the default values for pgoutput
options to improve clarity and usability.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/d2790f10-238d-4cb5-a743-d9d2a9dd900f@oss.nttdata.com