Clarify the purpose of the `write_only` argument of the `spacelift_environment_variable` resource type

Description of the change

Clarify the purpose of the write_only argument of the spacelift_environment_variable resource type.

[ ] Bug fix (non-breaking change that fixes an issue)
[ ] New feature (non-breaking change that adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[x] Documentation (non-breaking change that adds documentation)

None

[ ] Lint rules pass locally
[ ] The code changed/added as part of this pull request has been covered with tests
[ ] All tests related to the changed code pass in development
[ ] Examples for new resources and data sources have been added
[ ] Default values have been documented in the description (e.g., "Dummy: (Boolean) Blah blah. Defaults to false.)
[ ] If the action fails that checks the documentation: Run go generate to make sure the docs are up to date

[x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
[x] Pull Request is no longer marked as "draft"
[x] Reviewers have been assigned
[ ] Changes have been reviewed by at least one other engineer