Updated NamedPitch.diatonic_pitch_number API entry.

Added new examples.
Greatly extended NamedPitch API entry.
Added NamedPitch.absolute_staff_position. NEW.

The property is defined in terms of diatonic pitch number.

NEW: should be included in the 2.16 release notes.
Added NamedPitch.from_absolute_staff_position() method.

The method is currently a stub.

Implementation will follow.