bigtable: Change documentation on ReadModifyWrite and friends.
bigtable: Change documentation on ReadModifyWrite and friends.

These operations don't actually work how their names suggest they work.

Change-Id: I721a27e75a2f4a8e88e0f63bc6096527c49b99e0

roth1002/gcloud-golang,Maksadbek/gcloud-golang,googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,roth1002/gcloud-golang,googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,michael-grosshans/google-cloud-go,ptomasroos/gcloud-golang,googleapis/google-cloud-go,googleapis/google-cloud-go,blowmage/gcloud-golang,Maksadbek/gcloud-golang,michael-grosshans/google-cloud-go,blowmage/gcloud-golang,ptomasroos/gcloud-golang