Clarify comparison between ceil/expand and floor/trunc

[LilyFirefly]

Description

Clarify the comparison between ceil and expand and between floor and trunc. The previous wording implied (to me) that ceil and floor behaved the same for positive deltas (which is incorrect) and similarly for expand and trunc.

Checklist

• Build runs successfully
• Type stubs updated
• Docs updated
• If docstrings were affected, check if they appear correctly in the docs as well as autocomplete

Release checklist (maintainers only)

• Version updated in pyproject.toml
• Version updated in changelog
• Branch merged
• Tag created and pushed
• Confirm publish job runs successfully
• Github release created

[ariebovenberg]

Agree this is an improvement 👍 . Thanks for taking the time to submit this PR