doc: improve package install instructions

Including:
- additional clarification for Pg 9.x RPM package names
- consistent usage of sudo

doc/install-packages.sgml

@@ -29,9 +29,10 @@
   </para>
   <note>
     <para>
-      &repmgr; packages are designed to be compatible with the community-provided PostgreSQL packages.
+      &repmgr; RPM packages are designed to be compatible with the community-provided PostgreSQL packages
+      and 2ndQuadrant's <ulink url="https://www.2ndquadrant.com/en/resources/2ndqpostgres/">2ndQPostgres</ulink>.
       They may not work with vendor-specific packages such as those provided by RedHat for RHEL
-      customers, as the filesystem layout may be different to the community RPMs.
+      customers, as the PostgreSQL filesystem layout may be different to the community RPMs.
       Please contact your support vendor for assistance.
     </para>
   </note>
@@ -47,7 +48,6 @@
     <title>2ndQuadrant public RPM yum repository</title>
 
     <para>
-      Beginning with <ulink url="https://repmgr.org/docs/current/release-4.0.5.html">repmgr 4.0.5</ulink>,
       <ulink url="https://2ndquadrant.com/">2ndQuadrant</ulink> provides a dedicated <literal>yum</literal>
       <ulink url="https://dl.2ndquadrant.com/">public repository</ulink> for 2ndQuadrant software,
       including &repmgr;. We recommend using this for all future &repmgr; releases.
@@ -78,6 +78,14 @@
 	    <programlisting>
 curl https://dl.2ndquadrant.com/default/release/get/10/rpm | sudo bash</programlisting>
 	  </para>
+
+	  <para>
+	    For PostgreSQL 9.6 on CentOS, execute:
+	    <programlisting>
+curl https://dl.2ndquadrant.com/default/release/get/9.6/rpm | sudo bash</programlisting>
+	  </para>
+
+
 	  <para>
 	    Verify that the repository is installed with:
 	    <programlisting>
@@ -93,8 +101,23 @@ sudo yum repolist</programlisting>
           <para>
             Install the &repmgr version appropriate for your PostgreSQL version (e.g. <literal>repmgr10</literal>):
             <programlisting>
-$ yum install repmgr10</programlisting>
+sudo yum install repmgr10</programlisting>
           </para>
+          <note>
+            <para>
+              For packages for PostgreSQL 9.6 and earlier, the package name does not contain
+              a period between major and minor version numbers, e.g.
+              <literal>repmgr96</literal>.
+            </para>
+          </note>
+          <tip>
+            <para>
+              To determine the names of available packages, execute:
+              <programlisting>
+yum search repmgr</programlisting>
+            </para>
+          </tip>
+
         </listitem>
       </itemizedlist>
     </para>
@@ -163,7 +186,6 @@ $ yum install repmgr10</programlisting>
     <title>2ndQuadrant public apt repository for Debian/Ubuntu</title>
 
     <para>
-      Beginning with <ulink url="https://repmgr.org/docs/current/release-4.0.5.html">repmgr 4.0.5</ulink>,
       <ulink url="https://2ndquadrant.com/">2ndQuadrant</ulink> provides a
       <ulink url="https://dl.2ndquadrant.com/">public apt repository</ulink> for 2ndQuadrant software,
       including &repmgr;.
@@ -207,7 +229,7 @@ curl https://dl.2ndquadrant.com/default/release/get/deb | sudo bash</programlist
 	  <para>
             Install the &repmgr version appropriate for your PostgreSQL version (e.g. <literal>repmgr10</literal>):
             <programlisting>
-$ apt-get install postgresql-10-repmgr</programlisting>
+sudo apt-get install postgresql-10-repmgr</programlisting>
 	  </para>
           <note>
             <para>