Document semantics of upload to existing key.
Document semantics of upload to existing key.

Point to the 'object versioning' and 'lifetime' specs for details about
how to override the default (overwrite unconditionally).

Fixes #162.

tseaver/gcloud-python,lucemia/gcloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,dhermes/gcloud-python,jbuberel/gcloud-python,GrimDerp/gcloud-python,jgeewax/gcloud-python,VitalLabs/gcloud-python,waprin/gcloud-python,jonparrott/google-cloud-python,vj-ug/gcloud-python,EugenePig/gcloud-python,quom/google-cloud-python,daspecster/google-cloud-python,googleapis/google-cloud-python,daspecster/google-cloud-python,GrimDerp/gcloud-python,dhermes/google-cloud-python,dhermes/google-cloud-python,jbuberel/gcloud-python,vj-ug/gcloud-python,Fkawala/gcloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,quom/google-cloud-python,optimizely/gcloud-python,EugenePig/gcloud-python,googleapis/google-cloud-python,GoogleCloudPlatform/gcloud-python,tseaver/google-cloud-python,tseaver/gcloud-python,dhermes/gcloud-python,waprin/google-cloud-python,Fkawala/gcloud-python,tswast/google-cloud-python,jonparrott/gcloud-python,elibixby/gcloud-python,CyrusBiotechnology/gcloud-python,VitalLabs/gcloud-python,waprin/gcloud-python,elibixby/gcloud-python,tseaver/google-cloud-python,blowmage/gcloud-python,CyrusBiotechnology/gcloud-python,GoogleCloudPlatform/gcloud-python,waprin/google-cloud-python,optimizely/gcloud-python,lucemia/gcloud-python,blowmage/gcloud-python,optimizely/gcloud-python,jonparrott/google-cloud-python,tartavull/google-cloud-python,calpeyser/google-cloud-python,tartavull/google-cloud-python,jgeewax/gcloud-python,jonparrott/gcloud-python,thesandlord/gcloud-python,thesandlord/gcloud-python,calpeyser/google-cloud-python