The docs imply that one needs to use `dependencies` or `needs`
to gain variable inheritance in later jobs, but this isn't true. All jobs
in later stages will receive an inherited value without requiring
those keywords.

This change clarifies the documentation to reflect the observed
behavior. It also adds examples demonstrating how variable
inheritance can be turned off explicitly, and how inherited variables
may take precedence over other methods of variable definition.

The required format for the .env file is also explicitly stated, based
on its its implementation at:
https://gitlab.com/gitlab-org/gitlab/blob/2e5b4dbf3ab159f7ea4960d5fe7dfb06c7ed93e1/app/services/ci/parse_dotenv_artifact_service.rb#L55-58