webkit: Update documentation for onJsBeforeUnload callback

Fix documentation to clearly indicate that the default behavior is to
show WebView's own default dialog, and also describe the default
behavior more clearly and how to customize it.

Bug: 154014645
Test: m -j offline-sdk-docs seems not broken
Change-Id: I7d1e10c5d406ed739fb3963b9099791cfce95063
1 file changed