pgsql: Various fixes for the SGML docs.

Log Message:
-----------
Various fixes for the SGML docs. Consistently use spaces before/after
parentheses in syntax descriptions. Consistently use the present tense
when describing the basic purpose of each "DROP" command. Add a few
more hyperlinks.

Modified Files:
--------------
pgsql/doc/src/sgml:
catalogs.sgml (r2.151 -> r2.152)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml.diff?r1=2.151&r2=2.152)
pgsql/doc/src/sgml/ref:
alter_index.sgml (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_index.sgml.diff?r1=1.12&r2=1.13)
analyze.sgml (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/analyze.sgml.diff?r1=1.21&r2=1.22)
close.sgml (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/close.sgml.diff?r1=1.25&r2=1.26)
drop_aggregate.sgml (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_aggregate.sgml.diff?r1=1.30&r2=1.31)
drop_domain.sgml (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_domain.sgml.diff?r1=1.17&r2=1.18)
drop_language.sgml (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_language.sgml.diff?r1=1.25&r2=1.26)
drop_sequence.sgml (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_sequence.sgml.diff?r1=1.26&r2=1.27)
drop_table.sgml (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_table.sgml.diff?r1=1.26&r2=1.27)
drop_trigger.sgml (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_trigger.sgml.diff?r1=1.21&r2=1.22)
drop_type.sgml (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_type.sgml.diff?r1=1.29&r2=1.30)
execute.sgml (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/execute.sgml.diff?r1=1.14&r2=1.15)
pg_restore.sgml (r1.70 -> r1.71)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_restore.sgml.diff?r1=1.70&r2=1.71)
prepare.sgml (r1.22 -> r1.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/prepare.sgml.diff?r1=1.22&r2=1.23)
select.sgml (r1.99 -> r1.100)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select.sgml.diff?r1=1.99&r2=1.100)