Document await that it will never return if shutdown-agents was called

Undocumented behavior or the "await" function: it will never return if shutdown-agents was called.

This was a surprise to me to find yet another condition when await never returns.

Proposal: Change the docstring for clojure.core/await from "...Will never return if a failed agent is restarted with :clear-actions true." to
"...Will never return if a failed agent is restarted with :clear-actions true or shutdown-agents was called."

Patch: CLJ-1918.patch

Prescreened by: Alex Miller