Add section order check in util/find-doc-nits

This patch checks if the EXAMPLES section in a pod file is placed
before the RETURN VALUES section.

Reviewed-by: Richard Levitte <levitte@openssl.org>

(cherry picked from commit cc838ee2d66f7295bf7a7e6695aab1080d6791e9)

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8736)