[doc] add a note to visibility specification to avoid confusion
@@ -57,6 +57,8 @@ in `//tests/BUILD`, but not by targets defined in `//tests/integration/BUILD`.
As a special case, `package_group` targets themselves do not have a `visibility`
attribute; they are always publicly visible.
+Visibility cannot be set to specific non-package_group targets. That triggers a "Label does not refer to a package group" or "cycle in dependency graph" error.
+
## Visibility of a rule target
If a rule target does not set the `visibility` attribute, its visibility is
