Detail provider project requirements more explicitly in external docs

[ScottSuarez]

motivation

We regularly have users reach out to us, confused as to why they get plan time errors provisioning resources where they don't provide the project on the resource itself. Since they also don't define a provider level project as an environment variable or project value in the provider block, we have no source for the project.

The argument states since the field is optional this value shouldn't need to be provided. Unfortunately this field being optional is a misnomer. It is required, but technically optional since it's usually sourced from a whole bunch of different locations.

action

Clarify this misnomer at the documentation level. Linking how project sourcing works and the true requirements therein.

ref

• google_monitoring_notification_channel == project parameter is missing in documentation and it is required  #12470