Rework util/find-doc-nits to distinguish internal documentation

Rework util/find-doc-nits to distinguish internal documentation

We didn't really distinguish internal and public documentation, or
matched that with the state of the documented symbols.  we therefore
needed to rework the logic to account for the state of each symbol.

To simplify things, and make them consistent, we load all of
util/*.num, util/*.syms and util/missing*.txt unconditionally.

Also, we rework the reading of the manuals to happen only once (or
well, not quite, Pod::Checker reads from file too, but at the very
least, our script isn't reading the same file multiple times).

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11476)