Custom variable validation is a useful feature that is supported by
Nomad and not just Terraform. As such it should be documented on the
input variable page.
I've cribbed the content from the terraform docs so this should be
consistent across projects
Block labels are not expressions so they can't be interpolated without hacks
like `dynamic` blocks. Clarify this so that we don't confuse users who
shouldn't need to dig into the subtle nuances between expressions and blocks.
