Bring alias docstrings more in line with style guide.

[copybara-service]

Bring alias docstrings more in line with style guide.

This includes four types of changes:

• Capitalization of the first letter (previously this was inconsistent with
  lower case first letter being used more often). Now they all use capital
  letters like in the style guide.
• American spelling as required by the style guide.
• Remove type information from Args section in the docstring wherever this
  information is already provided by the type annotation.
• Remove default values from the Args section in the docstring if this
  information is already provided in the function signature.

The last two items are to avoid duplicate information, which runs the risk of
becoming wrong or inconsistent over time.