pgsql: SGML cleanup: consistently use "endterm" in <xref>s that link to

Log Message:
-----------
SGML cleanup: consistently use "endterm" in <xref>s that link to the
reference page for SQL commands, so that the link text is italicized.

Modified Files:
--------------
pgsql/doc/src/sgml:
ddl.sgml (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml.diff?r1=1.40&r2=1.41)
func.sgml (r1.253 -> r1.254)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.253&r2=1.254)
maintenance.sgml (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml.diff?r1=1.43&r2=1.44)
manage-ag.sgml (r2.40 -> r2.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/manage-ag.sgml.diff?r1=2.40&r2=2.41)
mvcc.sgml (r2.50 -> r2.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/mvcc.sgml.diff?r1=2.50&r2=2.51)
query.sgml (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/query.sgml.diff?r1=1.44&r2=1.45)
runtime.sgml (r1.324 -> r1.325)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.324&r2=1.325)
pgsql/doc/src/sgml/ref:
psql-ref.sgml (r1.142 -> r1.143)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.142&r2=1.143)