[WebView] Improve documentation around full screen mode.

onShowCustomView and onHideCustomView are only used for full screen
support so we make that more explicit now (perhaps we should consider
rename and deprecate).

In M40 the WebView will also support full screen for non <video>
elements so we also consider that use case.

Change-Id: Ie4e245defbdd3e53bff98ac73305dd5e5e46334c