This is currently called from a pre-send update, but most if what that
method did was alrady post-send. It does:
* check permissions (read query).
* run onUserClearNewTalkNotification hook.
* everything else is in a post-send deferred update.
The onUserClearNewTalkNotification hook in turn is only used by one
extension indexed by Codesearch, namely Echo, and all that handler
does is... schedule a post-send update.
* Improve onUserClearNewTalkNotification documentation.
* Explain why onPageViewUpdates is pre-send (for now).
Bug: T277788
Change-Id: Idb6e7524c32c6fbb8760a1b62091d493423aa51e