Updated docs w.r.t. infinite doc length changes

[rmitsch]

Description

Updated docs w.r.t. infinite doc length changes. See explosion/spacy-llm#332.

Also updates task list in usage docs with TranslationTask.

Types of change

Docs update.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[svlandeg]

Thanks, looks good!

Only had a few typo corrections, and two tables that need fixing in terms of formatting.