vis.3: Try to better describe VIS_SAFE

[des]

Summary

The current text is correct, but fails to draw the reader's attention
to the fact that VIS_SAFE essentially works in reverse, i.e. it exempts
certain characters from being encoded.

While here, fix some markup nits.

MFC after: 1 week

[des]

@adrian could you perhaps look into getting this reviewed and committed upstream?