pgsql: Add note that PREPARE TRANSACTION is for transaction managers,

Log Message:
-----------
Add note that PREPARE TRANSACTION is for transaction managers, not
regular applications. Also add a comment pointing out that tab-complition
for PREPARE TRANSACTION is missing on purpose.

Modified Files:
--------------
    pgsql/src/bin/psql:
        tab-complete.c (r1.191 -> r1.192)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c?r1=1.191&r2=1.192)
    pgsql/doc/src/sgml/ref:
        prepare_transaction.sgml (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/prepare_transaction.sgml?r1=1.9&r2=1.10)