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>
(Merged from https://github.com/openssl/openssl/pull/8729)