Doc change: no-cache cannot be used with client.mutate update

[hwillson]

Re-capping the doc change (since it outlines the issue):

Note that since this function is intended to be used to update the store, it cannot be used with a no-cache fetch policy. If you're interested in performing some action after a mutation has completed, and you don't need to update the store, use the Promise returned from client.mutate instead.

Fixes #3401.