Merge branch 'upstream'

Merge branch 'upstream'

Merges in the minor change golang/go@92c29ef

Unexport Pending field from CancelArgs

The intention of this is to make the cancel message smaller on the wire.
Previously `encoding/gob` would encode the empty `Pending` field onto the wire
for each cancel RPC. This was an oversight. Luckily we can just unexport it and
since the field was unused this change is both backwards and forwards
compatible.

Tested cancellations with the combinations `{old_client,
new_client}x{old_server, new_server}` using
https://gist.github.com/keegancsmith/a53fbd2d4849343d6ca70b27fd062667

Update docs to include motivation