translate: Make help more helpful about language :hints

[half-duplex]

Description

Don't make me PM the bot to not even be reminded how .tr wants language hints.

You may now discuss whether we should reorder the examples, have one or two hints, why we don't have a @plugin.help("Something less docstring-ish here"), etc. in comments.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

Re-requested review because I changed the patch to include both :hint and non-hint versions of the command, which served a double purpose in forcing GH to update the base branch for a new CI run (since it was stubbornly refusing to create a fresh merge for running tests after #2455 was merged).

Help output is now:

<dgw> .help tr
<SopelTest> dgw: Translates a phrase, with an optional language :hint.
<SopelTest> e.g. .tr mon chien or .tr :en :fr my dog

Optionally, @half-duplex might want to re-amend the commit with a proper signature, but given our (lack of a) project policy re: commit signing, it doesn't actually matter.