text: Title: Codify style for doctring summary lines
  Body: Currently, we ask contributors to strive for consistency with existing code, but it would be helpful to clarify the following regarding docstrings:
  
  - Docstrings should begin with a short (~70 characters or less) summary line that ends in a period.
  - The summary line should begin immediately after the opening quotes (do not add a line break before the summary line)
  - The summary line should describe what it *is* if it is a class (e.g., "An asynchronous, file-like object for reading ASGI streams.")
  - The summary line should describe what it *does* when called, if it is a function, structured as an imperative (e.g., "Delete a header that was previously set for this response.")
  
  We will need two patches to address this issue:
  
  - [x] Update CONTRIBUTING.md to include the guidelines above
  - [x] Fix up any existing docstrings to provide a baseline of consistency going forward