Bring documentation from python OpenTime library forward to the C++ methods and the docstrings

The original python implementation of OpenTimelineIO had docstrings on many of the functions and methods in opentime. We should bring these forward into the C++ and also the python bindings so that users have that information readily available.

documentation:
https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/last_pure_python/opentimelineio/opentime.py

Start with the original documentation in opentimelineio/opentime.py from the linked last_pure_python revision, then locate the corresponding C++ methods and Python bindings. The work is done when the relevant documentation is available in both the C++ APIs and their Python bindings.

Written by the indexing model from the issue text.