WebView: clarify docs for onShowCustomView

No change to logic, only docs.

This clarifies the docs for onShowCustomView. This @links to
FLAG_FULLSCREEN, reminds the developer they must override both
onShowCustomView and onHideCustomView, and provides guidance for
CustomViewCallback.

Bug: 143247282
Test: make -j4 docs
Change-Id: I64de3723674da5c138438921cc8232c4bf2a3d98