[llvm] [mlir][Bazel] Add a comment and move deps list into an extra line. (PR #93453)

[Adrian Kuegel via llvm-commits]

akuegel wrote:

An alternative idea that was suggested by @jpienaar was to create a build rule that returns the optional dependencies, and here in the LLVM repository it would return an empty list, but in our build we could let it return the optional dependency that we need. What do you think?

https://github.com/llvm/llvm-project/pull/93453