util/find-doc-nits: Better checking of missing documentation

util/find-doc-nits: Better checking of missing documentation

The names collected in util/missing*.txt are not file names, but
symbol names, and to compare properly with script data, the section
name must be included.

All symbols found in util/lib*.num are library functions, so we know
that they are in manual section 3 and can simply add that info.  The
same goes for all macros found in C headers.

Finally, we get rid of getdocced() and its associated hash table
%docced.  We already have the appropriate information in %name_map.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10621)