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.

doc/src/sgml/installation.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.352 2010/09/08 20:35:51 petere Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.353 2010/09/09 17:19:40 tgl Exp $ -->
 
 <chapter id="installation">
  <title><![%standalone-include[<productname>PostgreSQL</>]]>
@@ -97,9 +97,7 @@ su - postgres
      <para>
       <application>tar</> is required to unpack the source
       distribution, in addition to either
-      <application>gzip</> or <application>bzip2</>.  In
-      addition, <application>gzip</> is required to install the
-      documentation.
+      <application>gzip</> or <application>bzip2</>.
      </para>
     </listitem>
 
@@ -139,7 +137,7 @@ su - postgres
        <primary>zlib</primary>
       </indexterm>
 
-      The <productname>zlib</productname> compression library will be
+      The <productname>zlib</productname> compression library is
       used by default. If you don't want to use it then you must
       specify the <option>--without-zlib</option> option to
       <filename>configure</filename>. Using this option disables
@@ -237,7 +235,7 @@ su - postgres
 
     <listitem>
      <para>
-      If you want to build the <application>PL/Tcl</application>
+      To build the <application>PL/Tcl</application>
       procedural language, you of course need a <productname>Tcl</>
       installation.  If you are using a pre-8.4 release of
       <productname>Tcl</>, ensure that it was built without multithreading
@@ -266,10 +264,10 @@ su - postgres
 
     <listitem>
      <para>
-      <application>Kerberos</>, <productname>OpenSSL</>,
+      You need <application>Kerberos</>, <productname>OpenSSL</>,
       <productname>OpenLDAP</>, and/or
       <application>PAM</>, if you want to support authentication or
-      encryption using these services.
+      encryption using those services.
      </para>
     </listitem>
    </itemizedlist>