Zoned datetime tweaks

[voodoos]

This PR is splited in 3 commits:

• Adds the possibility to specify the timezone when creating a Zoned_datetime from a UTC datetime, in symmetry with symmetric of from_local_datetime
• Reword some doc comments to make it more explicit when an input datetime is expected to be in local time.
• Shallow doc comments style changes and uniformisation (third person everywhere, doesn't -> do not)

[gr-im]

Thanks ! I think the changes deserve a Change entry!

[voodoos]

Thank you, I added one in the last commit.