Clarify that midi_tempo only sets MIDI header tempo, not note timing.

[candpixie]

Closes #152.

Changed documentation only. Referenced Issue #152, which reports that midi_tempo doesn't
affect playback speed. It's correct: in note_events_to_midi, midi_tempo
is only passed to pretty_midi.PrettyMIDI(initial_tempo=...), which writes the
tempo into the MIDI header. Note start/end times come from the audio in
absolute seconds and are never rescaled by midi_tempo, so it cannot change
timing or duration but only the header value.

This PR helps clarify that in the --midi-tempo CLI help and in the docstrings for
note_events_to_midi, predict, and predict_and_save (which previously
omitted midi_tempo). Also, no behavior changes have been made.