Update documentation to reflect deprecation of "NewEnvClient"
Update documentation to reflect deprecation of "NewEnvClient"

As `NewEnvClient` is deprecated in favor of `NewClientWithOpts`, the main package documentation should reflect this. This is also the text that appears on godoc.org so it's quite important that it is correct (for newbies like me)

Signed-off-by: Tobias Pfandzelter <aa048cf5da7f2c8e6f98620ad42998abdc2682de@campus.tu-berlin.de>

thaJeztah/docker,docker/docker,justincormack/docker,Ramzec/docker,nawawi/docker,nalind/docker,gnailuy/docker,estesp/docker,trebonian/docker,Mokolea/docker,cpuguy83/docker,docker/docker,djs55/docker,mahak/docker,jameszhan/docker,konstruktoid/docker-upstream,konstruktoid/docker-upstream,yosifkit/docker,AkihiroSuda/docker,flavio/docker,justincormack/docker,yosifkit/docker,nawawi/docker,djs55/docker,moul/docker,Mokolea/docker,jimjag/docker,Mokolea/docker,Mokolea/docker,estesp/docker,yosifkit/docker,mahak/docker,gnailuy/docker,flavio/docker,Ramzec/docker,kojiromike/docker,ibuildthecloud/docker,konstruktoid/docker-upstream,bearstech/moby,ibuildthecloud/docker,jimjag/docker,trebonian/docker,moul/docker,moul/docker,bearstech/moby,dims/docker,ibuildthecloud/docker,cpuguy83/docker,paralin/docker,AkihiroSuda/docker,moby/moby,yosifkit/docker,jthelin/docker,nalind/docker,jameszhan/docker,nalind/docker,kojiromike/docker,vdemeester/docker,jameszhan/docker,bearstech/moby,ibuildthecloud/docker,Ramzec/docker,konstruktoid/docker-upstream,mahak/docker,moul/docker,flavio/docker,moby/moby,docker/docker,thaJeztah/docker,trebonian/docker,gnailuy/docker,jthelin/docker,djs55/docker,paralin/docker,dims/docker,paralin/docker,jimjag/docker,flavio/docker,mahak/docker,nawawi/docker,thaJeztah/docker,nawawi/docker,paralin/docker,AkihiroSuda/docker,nalind/docker,AkihiroSuda/docker,Ramzec/docker,trebonian/docker,vdemeester/docker,moby/moby,ibuildthecloud/docker,jimjag/docker,cpuguy83/docker,cpuguy83/docker,bearstech/moby,kojiromike/docker,dims/docker,thaJeztah/docker,estesp/docker,justincormack/docker,vdemeester/docker,moby/moby,gnailuy/docker