Clarify registration of characters in Rests range

[dspreadbury]

In steinbergmedia/bravura#8 @ewanmellor expresses confusion over the vertical placement of the whole rest glyph (restWhole, U+E4E3). We should clarify in the Implementation notes for the Rests range of characters in the specification that y = 0 corresponds to a staff line, but not necessarily a specific staff line, so the half rest sits on top of y = 0 while the whole rest hangs from y = 0.