doc: add notes about package compatibility

We need to emphasise that the repmgr packages are only compatible
with packages based on the PGDG filesystem layout; 3rd party vendor
packages often put application and data directories elsewhere.
See e.g. GitHub #427.

doc/appendix-faq.sgml

@@ -87,12 +87,27 @@
    </para>
    <para>
      To minimize downtime during major upgrades, for more recent PostgreSQL
-     versions <ulink url="https://www.2ndquadrant.com/en/resources/pglogical/">pglogical</ulink>
+     versions (PostgreSQL 9.4 and later),
+     <ulink url="https://www.2ndquadrant.com/en/resources/pglogical/">pglogical</ulink>
      can be used to set up a parallel cluster using the newer PostgreSQL version,
      which can be kept in sync with the existing production cluster until the
      new cluster is ready to be put into production.
    </para>
   </sect2>
+
+  <sect2 id="faq-libdir-repmgr-error">
+   <title>What does this error mean: <literal>ERROR: could not access file "$libdir/repmgr"</literal>?</title>
+   <para>
+     It means the &repmgr; extension code is not installed in the
+     PostgreSQL application directory. This typically happens when using PostgreSQL
+     packages provided by a third-party vendor, which often have different
+     filesystem layouts.
+   </para>
+   <para>
+     Either use PostgreSQL packages provided by the community or 2ndQuadrant; if this
+     is not possible, contact your vendor for assistance.
+   </para>
+  </sect2>
  </sect1>
 
  <sect1 id="faq-repmgr" xreflabel="repmgr">

doc/appendix-packages.sgml

@@ -20,18 +20,23 @@
       Currently, &repmgr; RPM packages are provided for versions 6.x and 7.x of CentOS. These should also
       work on matching versions of Red Hat Enterprise Linux, Scientific Linux and Oracle Enterprise Linux;
       together with CentOS, these are the same RedHat-based distributions for which the main community project
-      provides packages (see the <ulink url="https://yum.postgresql.org/">PostgreSQL RPM Building Project</ulink>
+      (PGDG) provides packages (see the <ulink url="https://yum.postgresql.org/">PostgreSQL RPM Building Project</ulink>
       page for details).
     </para>
-    <para>
+
-      It appears Red Hat provides enterprise users with PostgreSQL RPMs with a different
-      filesystem layout to the community RPMs, in which case the 2ndQuadrant &repmgr; RPM packages
-      may not be compatible. Please contact your support vendor for assistance.
-    </para>
     <para>
       Note these &repmgr; RPM packages are not designed to work with SuSE/OpenSuSE.
     </para>
 
+    <note>
+      <para>
+        &repmgr; packages are designed to be compatible with community-provided PostgreSQL packages.
+        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.
+        Please contact your support vendor for assistance.
+      </para>
+    </note>
+
     <sect2 id="packages-centos-repositories">
       <title>CentOS repositories</title>
 

doc/install-packages.sgml

@@ -26,6 +26,15 @@
    at the same time as each &repmgr; release, as it can take some days for
    them to become available via the main PGDG repository. See following section for details:
   </para>
+  <note>
+    <para>
+      &repmgr; packages are designed to be compatible with the community-provided PostgreSQL packages.
+      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.
+      Please contact your support vendor for assistance.
+    </para>
+  </note>
+
   <para>
     For more information on the package contents, including details of installation
     paths and relevant <link linkend="configuration-service-commands">service commands</link>,