command: Remove promise of plan -validate-only from validate docs
command: Remove promise of plan -validate-only from validate docs

We brought forward a new implementation of "terraform validate" that was
originally scheduled for a later release after finding that it would be
simpler than reworking the old implementation for new v0.12 assumptions,
but we didn't yet implement "terraform plan -validate-only" in spite of
it being mentioned in the updated docs for "terraform validate".

For now then, the documentation will make the weaker suggestion of running
"terraform plan" to validate a particular _run_ rather than a particular
_module_, which is the closest thing we have for now. At some point after
v0.12.0 we will evaluate whether a validate-only mode for "terraform plan"
(which could then run without configuring the providers at all) is needed.

pradeepbhadani/terraform,pradeepbhadani/terraform,pdecat/terraform,brandontosch/terraform,hashicorp/terraform,pdecat/terraform,luis-silva/terraform,apparentlymart/terraform,luis-silva/terraform,heimweh/terraform,tommynsong/terraform,VladRassokhin/terraform,apparentlymart/terraform,nlamirault/terraform,nlamirault/terraform,ewbankkit/terraform,rnaveiras/terraform,hashicorp/terraform,drebes/terraform,nlamirault/terraform,gechr/terraform,pdecat/terraform,pdecat/terraform,pryorda/terraform,luis-silva/terraform,pradeepbhadani/terraform,heimweh/terraform,rnaveiras/terraform,ewbankkit/terraform,VladRassokhin/terraform,VladRassokhin/terraform,gechr/terraform,apparentlymart/terraform,drebes/terraform,brandontosch/terraform,hashicorp/terraform,pryorda/terraform,rnaveiras/terraform,heimweh/terraform,brandontosch/terraform,drebes/terraform,heimweh/terraform,tommynsong/terraform,ewbankkit/terraform,gechr/terraform,brandontosch/terraform,apparentlymart/terraform,apparentlymart/terraform,heimweh/terraform,ewbankkit/terraform,pryorda/terraform,tommynsong/terraform,luis-silva/terraform,VladRassokhin/terraform,brandontosch/terraform,drebes/terraform