Update: Document what Full Width does to the trickle button (fixes #265)

[swashbuck]

Fixes #265

Update

• Retitled the Full Width option so it says what it mainly does — dock the button to the bottom of the window — instead of describing only the width.
• Added help text to it covering both of the settings it takes over: step locking is forced on, and the button is always hidden after completion whatever the author picked for visibility after completion.
• Added help text to the step locking Enabled option saying it is forced on when the button is docked, so the override is visible from either side.

Text only — no behaviour change, and the _isFullWidth key is untouched, so nothing needs a migration. Renaming the key would be breaking; this is the non-breaking half.

Same treatment #260 gave _styleAfterClick, applied to the option that actually causes the overrides. Wording follows that PR: sentence, Default: x., then the caveat. Both schema formats updated, and the cross-references use the label each format displays.

Testing

1. Open a course in the authoring tool and edit a block or article with Trickle enabled.
2. Under the Trickle button settings, check the option formerly labelled "Make full width" now reads as docking to the bottom of the window and shows help text naming the two settings it overrides.
3. Check the step locking Enabled option shows help text saying it is forced on when the button is docked.
4. No framework-side testing needed — no behaviour changed. Worth a quick confirm that both schema files still parse and the Trickle settings render without error.