pgsql: Remove obsolete claim that gzip is needed while installing PG's

Log Message:
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Remove obsolete claim that gzip is needed while installing PG's documentation.
It isn't, now that we ship the docs as loose files rather than a sub-tarball.

Also adjust the wording in a couple of places to make the lists of required
software read more consistently.

Tags:
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REL9_0_STABLE

Modified Files:
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pgsql/doc/src/sgml:
installation.sgml (r1.348.2.4 -> r1.348.2.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.348.2.4&r2=1.348.2.5)