postgres_fdw: Update comments in make_new_connection().

postgres_fdw: Update comments in make_new_connection().

Expand the comment about the parallel_commit option to mention that the
default is false.

Also, since the comment about alteration of the keep_connections option,
which was located above the expanded comment, holds true for the
parallel_commit option, rewrite it to reflect this, and move it to after
the expanded comment.

Follow-up for commit 04e706d42.

Discussion: https://postgr.es/m/CAPmGK16Kg2Bf90sqzcZ4YM5cN_G-4h7wFUS01qQpqNB%2B2BG5_w%40mail.gmail.com