Add note in pg_rewind documentation about read-only files

When performing pg_rewind, the presence of a read-only file which is not
accessible for writes will cause a failure while processing.  This can
cause the control file of the target data folder to be truncated,
causing it to not be reusable with a successive run.

Also, when pg_rewind fails mid-flight, there is likely no way to be able
to recover the target data folder anyway, in which case a new base
backup is the best option.  A note is added in the documentation as
well about.

Reported-by: Christian H.
Author: Michael Paquier
Reviewed-by: Andrew Dunstan
Discussion: https://postgr.es/m/20180104200633.17004.16377%40wrigleys.postgresql.org

doc/src/sgml/ref/pg_rewind.sgml

@@ -95,6 +95,26 @@ PostgreSQL documentation
    are currently on by default.  <xref linkend="guc-full-page-writes"/>
    must also be set to <literal>on</literal>, but is enabled by default.
   </para>
+
+  <warning>
+   <para>
+    If <application>pg_rewind</application> fails while processing, then
+    the data folder of the target is likely not in a state that can be
+    recovered.  In such a case, taking a new fresh backup is recommended.
+   </para>
+
+   <para>
+    <application>pg_rewind</application> will fail immediately if it finds
+    files it cannot write directly to.  This can happen for example when
+    the source and the target server use the same file mapping for read-only
+    SSL keys and certificates.  If such files are present on the target server
+    it is recommended to remove them before running
+    <application>pg_rewind</application>.  After doing the rewind, some of
+    those files may have been copied from the source, in which case it may
+    be necessary to remove the data copied and restore back the set of links
+    used before the rewind.
+   </para>
+  </warning>
  </refsect1>
 
  <refsect1>