Make doc/man7/ and doc/internal/man3/ conform with man-pages(7)

Make doc/man7/ and doc/internal/man3/ conform with man-pages(7)

It's all in the details, from man-pages(7):

    Formatting conventions for manual pages describing functions

        ...
        Variable names should, like argument names, be specified in italics.
        ...

    Formatting conventions (general)

        ...
        Special macros, which are usually in uppercase, are in bold.
        Exception: don't boldface NULL.
        ...

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10034)