Update WebBackForwardList javadoc.

Update javadoc for WebBackForwardList and WebHistoryItem. Remove
references to history items being updated as this no longer occurs, and
clarify in clone() that there's no need to acutally do this any more.

Also remove a bunch of implementation notes about
atomicity/synchronisation that are being rendered attached to the return
value and likely aren't even true any more, let alone relevant to
developers.

Bug: 74593032
Test: make offline-sdk-docs
Change-Id: I4ce33133d2ff18e7376d768c7a2e076ecd2a8b95