ad_sthlp : allow `_` in italic word

[kbjarkefur]

_regex_string_ is valid markdown, but in ad_sthlp is will be converted to {it:regex}string{it:

[kbjarkefur]

In this commit this was solved for hyperlinks.

Converting {it:regex}string back to _regex_string_ is straightforward and the commit linked to above provides some utilities for this. However, the tricky part is to precisely identify when to convert back so we do not convert back more cases when we should.