Document PillarboxPlayer APIs as supported on the main thread only

[defagos]

Description

This PR adds an important warning to PillarboxPlayer main documentation entry point, explicitly stating that corresponding APIs are supported from the main thread only. It also ensures that tracker lifecycle methods are called on the main thread, which was not guaranteed.

Calling PillarboxPlayer APIs from background threads is theoretically possible but requires some work, see #931. Until then it is best to warn developers about the current limitations, which should not change anything in practice (most public Pillarbox APIs are meant to be used from UI code anyway).

Changes made

• Add documentation warning.
• Ensure tracker lifecycle methods are called on the main thread.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).