Add l3doc option to not replace @@ in the PDF

[PhelypeOleinik]

expl3 macro names are in general longer, and writing the code so that it doesn't produce overfull boxes when typeset gets trickier, especially because the @@ are replaced by the package/module prefix, which makes the columns in the actual code not correspond to the columns in the typeset PDF.

For smaller modules and packages, in which the prefix is always the same, l3doc could have an option to disable the @@ replacement to shorten code lines.

[josephwright]

Duplicate of #416