improve documentation for music21.stream.Stream.hasElement()
improve documentation for music21.stream.Stream.hasElementsOfClass()
improve documentation for music21.stream.Stream.mergeElements()
improve documentation for music21.stream.Stream.index()
improve documentation for music21.stream.Stream.remove()
improve documentation for music21.stream.Stream.pop()
improve documentation for music21.stream.Stream.removeByClass()