Datetime: Tidy up docs

[StanFromIreland]

Fix some style guide infringements (case of titles, Latin abbreviations), add parameter mark up in a few places, fix a small typo, make spacing of docs consistent, and add an inheritance diagram (like pathlib's :-).

Diagram

Generated with graphviz, and:

digraph {
    graph [
        bgcolor="transparent"
        fontnames = "svg"
        layout="dot"
        ranksep=0.5
        nodesep=0.5
        splines=line
    ]
    node [
        fontname="Courier"
        fontsize=14.0
        shape=box
        style=rounded
        margin="0.15,0.07"
    ]
    edge [
        arrowhead=none
    ]
    object -> tzinfo
    object -> timedelta
    object -> time
    object -> date
    tzinfo -> timezone
    date -> datetime
}

---

📚 Documentation preview 📚: https://cpython-previews--144720.org.readthedocs.build/en/144720/library/datetime.html

[AA-Turner]

The diagram in pathlib is useful as the inheritance is non-standard. I don't think that's the case here, perhaps the only such instance is date <- datetime. What's the justification for adding it?

[StanFromIreland]

What's the justification for adding it?

The diagram is already in the documentation, albeit in textual form.

[AA-Turner]

feedback: this PR could have been more usefully structured as an optional set of style guide changes, addition of markup, and adding an image; to allow discussing each on its own merits.

Probably not worth doing now that this has been opened, but please keep in mind for the future.

A

[StanFromIreland]

Thanks Hugo!