Manual: "bugs" section

[Graham Percival]

Some notation elements have a little "bugs" section, like this one for 
micro tones:
---
There are no generally accepted standards for denoting three quarter 
flats, so LilyPond's symbol does not conform to any standard.
---

IMO this isn't really a bug.  A bug is something like "our current slur 
code sometimes produces ugly results.  This will be fixed at some 
point, but not in the near future" (paraphrased from the 1.x manuals).  
Since there aren't any standards for three quarter flats, I'd say that 
this warning is a... well, a warning.  Not a bug.  :)

Actually, I like that word.  (when I began writing this email, I didn't 
know what to call a non-bug thing that we needed to mention in the 
manual)

Could we add a @refwarning tag (like @refbug and @seealso) ?  There are 
some things that we _should_ draw attention to (such as this case, or 
the MIDI note about using sidestick instead of rim shot), but which 
aren't actually bugs.

Cheers,
- Graham