docs(models): explain thinking visibility vs model support

[webwww123]

What does this PR do?

Fixes #4803.

Adds a short "Thinking toggle behavior" section to model docs clarifying:

• /thinking controls visibility only
• actual reasoning depends on model/provider/variant support
• for Copilot, available reasoning behavior depends on selected model and plan
• ctrl+t can be used to cycle variants

How did you verify your code works?

• Aligned wording with existing /thinking and variant behavior documented across TUI/models docs.
• Validation will run in GitHub Actions CI.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.