mark placement attribute for <arpeggiate> and <non-arpeggiate> as unused

[lemzwerg]

As discussed in #540, the placement attribute of both the <arpeggiate> and the <non-arpeggiate> elements doesn't do anything and should thus be documented as unused, becoming obsolete in later versions.

[mscuthbert]

It's been exactly a month since this proposal was put forward, and I can't think of a (semantic) reason for having placement on these elements, and no one has come to their rescue, so I will mark this with the new label "Accepted-Awaiting PR" and keep it open while the DocGenerator refactor continues. Thanks Werner!