util/find-doc-nits: Relax check of function declarations in name_synopsis()

util/find-doc-nits: Relax check of function declarations in name_synopsis()

The relaxation allows spaces between function name and argument list,
to allow line breaks like this when there are very long names:

    int (fantastically_long_name_breaks_80char_limit)
        (fantastically_long_name_breaks_80char_limit *something);

This revealed some other intricaties, such as documented internal
structures with function pointers inside, so a check of open
structures was also added, and they are now simply skipped over.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12494)