Add docstring to `bazel sync --experimental_repository_resolved_file` output

### Description of the problem / feature request:

The `bazel sync --experimental_repository_resolved_file` output should include a module docstring.

### Feature requests: what underlying problem are you trying to solve with this feature?

The resolved file currently fails the [`buildifier` module-docstring`](https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#module-docstring) lint check.

### What operating system are you running Bazel on?

Linux, macOS

### What's the output of `bazel info release`?

`release 4.2.1`

### Have you found anything relevant by searching the web?

- bazelbuild/bazel#14454 (this issue is about formatting, this issue is about missing content)

Start by tracing the output produced by `bazel sync --experimental_repository_resolved_file` and review the linked Buildifier `module-docstring` lint check. The work is done when the resolved file includes a module docstring and no longer fails that lint check.

Written by the indexing model from the issue text.