gh-1002: Consistent newline at end of numpydoc docstrings

[paddyroddy]

Description

We're using the numpydoc specification but we weren't consistently adding the newline. This PR adds the newline.

Closes: #1022

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[connoraird]

Nice and cleaner