Clarify LTX2 pipeline docs and examples

[wunianze666-netizen]

Summary

• fix the LTX2ConditionPipeline example to unpack both video and audio when return_dict=False
• replace remaining LTX2 docstring TODOs with concrete descriptions
• correct outdated Gemma/T5 argument docs and a couple of LTX2 typos

Why

These LTX2 pipeline docs still contain placeholder text and one broken example snippet, which makes the new pipelines harder to follow.

Validation

• python -m py_compile on the touched LTX2 pipeline modules
• git diff --check

[github-actions]

Hi @wunianze666-netizen, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.