Give a better warning when using gazelle_override that this will break downstream packages

[Martin-Idel]

I had an issue which I seemingly could solve with using gazelle_override in a library package. However, a couple of weeks later when the library was used, I ran into this problem here: #1543

I don't want to dispute this - but I think there should be some form of prominent warning that using gazelle_override will break downstream packages - at least in the documentation (e.g. here: https://github.com/bazel-contrib/bazel-gazelle/blob/99af6b381f814a11f2042c1324c016aa136ea31f/extensions.md) and maybe even as build warning.

---

Just FYI why I wanted to use an override:

I wanted to build api-linter (https://github.com/googleapis/api-linter), which requires bufbuild/protocompile (https://github.com/bufbuild/protocompile) which does things with well-known imports overriding some other imports (bufbuild/protocompile#310) and results in me getting errors like

link: package conflict error: github.com/bufbuild/protocompile: package imports google.golang.org/protobuf/types/pluginpb
          was compiled with: @@gazelle++go_deps+com_github_bufbuild_protocompile//wellknownimports/google/protobuf/compiler:pluginpb
        but was linked with: @@gazelle++go_deps+org_golang_google_protobuf//types/pluginpb:pluginpb
See https://github.com/bazelbuild/rules_go/issues/1877.