Fix some inconsistencies with GUC categories

Fix some inconsistencies with GUC categories

This commit addresses a few things around GUCs:
- The TCP-related parameters (the four tcp_keepalives_* and
client_connection_check_interval are listed in postgresql.conf.sample in
a subsection called "TCP settings" of "CONNECTIONS AND AUTHENTICATION",
but they did not have their own group name in guc.c.
- enable_group_by_reordering, stats_fetch_consistency and
recovery_prefetch had an inconsistent description, missing a dot at the
end.
- In postgresql.conf.sample, "Process title" should not have a section
of its own, but it should be a subsection of "REPORTING AND LOGGING".

This impacts the contents of pg_settings, which could be seen as a
compatibility break, so no backpatch is done.  This is similar to the
cleanup done in a55a984.

Author: Shinya Kato
Discussion: https://postgr.es/m/5e0c9c608624eafbba910c344282cb14@oss.nttdata.com