doc: Rewrite how to get the source code

Instead of hardcoding a specific link, give a general link to the
download section of the web site.  This gives the user more download
options and the sysadmins more flexibility.  Also, the previously
presented link didn't work for devel versions.

doc/src/sgml/installation.sgml

@@ -363,16 +363,18 @@ su - postgres
   <title>Getting The Source</title>
 
   <para>
-   The <productname>PostgreSQL</> &version; sources can be obtained by
-   anonymous FTP from <ulink
-   url="ftp://ftp.postgresql.org/pub/source/v&version;/postgresql-&version;.tar.gz"></ulink>.
-   Other download options can be found on our website:
-   <ulink url="http://www.postgresql.org/download/"></ulink>. After you
-   have obtained the file, unpack it:
+   The <productname>PostgreSQL</> &version; sources can be obtained from the
+   download section of our
+   website: <ulink url="http://www.postgresql.org/download/"></ulink>.  You
+   should get a file named <filename>postgresql-&version;.tar.gz</filename>
+   or <filename>postgresql-&version;.tar.bz2</filename>. After
+   you have obtained the file, unpack it:
 <screen>
 <userinput>gunzip postgresql-&version;.tar.gz</userinput>
 <userinput>tar xf postgresql-&version;.tar</userinput>
 </screen>
+   (Use <command>bunzip2</command> instead of <command>gunzip</command> if you
+   have the <filename>.bz2</filename> file.)
    This will create a directory
    <filename>postgresql-&version;</filename> under the current directory
    with the <productname>PostgreSQL</> sources.