Make ASN1 manuals conform with man-pages(7)

Make ASN1 manuals conform with man-pages(7)

Details from man-pages(7) that are used:

    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: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10042)