webkit: Update documentation for onJsConfirm, onJsPrompt callbacks

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

Note that onJsBeforeUnload is not updated at the moment as I could not
find a way to reproduce it.

Bug: 154014645
Test: m -j offline-sdk-docs seems not broken

Change-Id: I5ee09ea35340eb8d17353eda1786dcebcff4a29e