convert-ly warnings too global

[Werner LEMBERG]

If I do

```
convert-ly en/notation/rhythms.itely > /dev/null
```

I get the following two warnings.


```
Not smart enough to convert music following \fine.

Warning: \fine no longer enforces the end of the music.  If your piece
has music following \fine that you want to exclude when it is
unfolded, use \volta to exclude it.
Please refer to the manual for details, and update manually.

Not smart enough to convert BreathingSign to Divisio.

MensuralStaff and VaticanaStaff now use Divisio_engraver to engrave
divisiones as Divisio grobs.

Previously, the respective Voice-aliased contexts used
Breathing_sign_engraver to engrave divisiones as BreathingSign grobs.
Because of the new names and the move from Voice to Staff, layout
adjustments for the old scheme are not effective for the new.

If you are not content with the new default layout, deal with Divisio
and Divisio_engraver in the appropriate Staff-aliased context.
Please refer to the manual for details, and update manually.
```


AFAICS, these warnings are unfounded; they are rather artifacts of
`convert-ly`, which applies regular expressions 'too globally'.

Is there any chance to fix this (i.e., improve the heuristics)?

BTW, a similar issue happens if I process
`en/notation/changing-defaults.itely` with `convert-ly`:

```
Not smart enough to convert BreathingSign to BarLine or Divisio.
```


    Werner