Pointer lock updated with clarification of lock and focus requirements.

After discussion with Olli Pettay I have clarified portions of the pointer
lock specification.

https://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html
https://dvcs.w3.org/hg/pointerlock/diff/6543d83c0b74/index.html

When requesting pointer lock, this paragraph replaces previous statements
regarding the target being in the DOM tree.
"""
Pointer lock must succeed only if the target is in the active document of a
browsing context which is (or has an ancestor browsing context which is) in
focus by a window which is in focus by the operating system's window
manager. The target element and its browsing context need not be in focus.
"""

In Requirements, clarification was added that focus MAY be shifted out of a
document and into others without exiting lock:
"""
The Pointer Lock API must exit the pointer lock state if the target is
removed from its document, or the user agent, window, or tab loses focus.
Moving focus between elements of active documents, including between
browsing contexts, does not exit pointer lock. E.g. using the keyboard to
move focus between contents of frames or iframes will not exit.
"""

Feedback and suggestions welcome.