Test and document list returning behavior for action functions

[jacobwhall]

• Adds tests/test_walk.py, which tests the behavior I document below
• Adds the following rule for action functions to docs/source/code.rst:

  If the received element is a block or inline element, they may return a list of elements of the same base class, which will take the place of the received element.

• Various spelling and capitalization fixes to documentation

We may want to consider allowing lists of MetaValues to be returned, though in my opinion it makes more sense to return a MetaList in those situations.

In #199, @ickc suggested that if action functions return a single element, that element should always be in a list, and that we should only allow standalone elements to be returned for compatibility reasons. Because of the difference between Block/Inline and MetaValue lists I described above, I'm not sure what would be best. I tried to write this documentation change to best reflect the current behavior of Panflute.

As a new contributor to this project (and in general), I welcome any feedback! Thank you for your time.

[jacobwhall]

@sergiocorreia It looks like the pytest step failed for the build GitHub Action for this PR. I'd be happy to help look into it.

[ickc]

@jacobwhall, the failure may be related to compatibility issues with newer pandoc versions.

But I wonder why this pull request doesn't has a GitHub Actions build?