docs: ZIP-related tweaks

[d-v-b]

Following up on #1615, this PR changes instances of "zip file" to "ZIP archive", "zipped zarr" to "ZIP-archived Zarr array or group" (following the conventional description of ZIP archives as such). I also added an explicit statement about when caching may be useful in the s3 + zip example. There's also some minor stylistic changes to the content added in #1615

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

given the changes I made, the CI failure is interesting but probably not for this PR

[d-v-b]

I also fixed some markup errors and typos in the release notes.

[joshmoore]

Thanks, @d-v-b!