Make check_example_location() in find-doc-nits generic

Change to check_section_location(), a generic function to ensure that
section SECTION appears before section BEFORE in the man pages.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>

(cherry picked from commit 95f92d57755a9bfc83135a585da69d497f7293d9)

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