datastore: Re-word requirement on f in RunInTransaction.
datastore: Re-word requirement on f in RunInTransaction.

Change-Id: I0e7943e727d8ef844ddf35df7f783c8a0f2a8d34
Reviewed-on: https://code-review.googlesource.com/7711
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go