Document return type for _read_frame

[janbridley]

Description

The return type of the _read_frame function was added to the Returns docstring block to eliminate a warning raised by type hinters.

Motivation and Context

Python type linters would make the incorrect assumption that __getitem__ from a trajectory always returned a HoomdTrajectoryView, even through __getitem__ with an int returns a gsd frame. This would lead to warnings like Cannot access attribute "particles" for class "_HOOMDTrajectoryView" when attempting to access properties that do not exist in a trajectory view. Updating the return type in the documentation of _read_frame fixes this issue.

How Has This Been Tested?

The warning is no longer raised.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the GSD Contributor Agreement.
• My name is on the list of contributors (doc/credits.rst) in the pull request source branch.
• I have added a change log entry to CHANGELOG.rst.

[joaander]

Thanks!