docs: Mention that gazelle:prefix can be replaced by go.mod

docs/go/core/bzlmod.md

@@ -75,11 +75,12 @@ If you have a use case that would require this, please explain it in an issue.
 
 ## Generating BUILD files
 
-Add the following to your top-level BUILD file, including your Go module's path in a [Gazelle directive](https://github.com/bazelbuild/bazel-gazelle#directives):
+Add the following to your top-level BUILD file, including your Go module's path in a [Gazelle directive](https://github.com/bazelbuild/bazel-gazelle#directives) if there is no `go.mod` file with that information in the same directory:
 
 ```starlark
 load("@gazelle//:def.bzl", "gazelle")
 
+# The next line is only needed without a go.mod file.
 # gazelle:prefix github.com/example/project
 gazelle(name = "gazelle")
 ```