pg_rewind option clarification

This patch clarifies the documentation for when --source-pgdata and
--source-server can be used.

I would like to apply this to 9.6.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
+                     Ancient Roman grave inscription +