bug#47408: Etags support for Mercury [v0.5]

[Francesco Potortì]

>> Yes, unless I am mistaken, ctags has no practical use outside of the
>>> C-family language domain.
>> That's difficult to tell for me.  However, etags provides ctags support
>> for all languages it knows about.  I may be missing something, but I
>> think Mercury would be the first exception.  If that's the case, maybe
>> it would be worth  adding a comment telling so.
>
>ctags only differs from etags by the fact that its output is 
>Vim-compatible. But it so happens that Mercury has superior in-built 
>support for Vim tagging (the core team use Vim). So ctags would not 
>actually be used by users with Vim-compatibility requirements, they 
>would just use Mercury tagging on Vim.

Maybe adding this info in a comment inside etags' code would be useful
for future maintainers.