Docs: better deprecation text

Expand the text on deprecation to be more descriptive and to refer
back to openssl_user_macros(7).

Incidently, this required a small change in util/find-doc-nits, to
have it skip over any line that isn't part of a block (i.e. that
hasn't been indented with at least one space.  That makes it skip over
deprecation text.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7745)