a_strex.c: improve documentation of 'tag2nbyte' lookup table

The 'tag2nbyte' lookup table maps the tags of ASN1 string types
to their respective character widths. It is used for example by
ASN1_STRING_to_UTF8(). This commit adds the tag names as comments.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6062)
