Test code snippets in docstrings

[pbrubeck]

Ideally we should be testing code snippets so they don't bitrot. This includes the ones under the examples in the docstrings. One way to test them is with doctest, which also needs the output from each example in the docstring, but this might not be ideal if the output is non-numeric.

[connorjward]

One case where this needs fixing is the Submesh docstring from #5004