pgsql: Document the existence of the socket lock file under

Log Message:
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Document the existence of the socket lock file under unix_socket_directory,
which is perhaps not a terribly good spot for it but there doesn't seem to be
a better place.  Also add a source-code comment pointing out a couple reasons
for having a separate lock file.  Per suggestion from Greg Smith.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.308 -> r1.309)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.308&r2=1.309)
    pgsql/src/backend/libpq:
        pqcomm.c (r1.212 -> r1.213)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c?r1=1.212&r2=1.213)