Incorrect documentation about `sysroot` extension.

[asoffer]

https://github.com/bazel-contrib/toolchains_llvm/blob/43e5dc41535c0f8d4f3573c7373caf10e883e4bb/toolchain/extensions/llvm.bzl#L121C17-L121C118

As best I can tell, this comment is incorrectly documenting the current behavior in which an empty set of targets means the sysroot definition is entirely ignored.