Incorrect documentation about `sysroot` extension.

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.

Review toolchain/extensions/llvm.bzl at the linked comment around line 121 and compare its wording with the current sysroot behavior when the target set is empty. Update the documentation to describe that behavior accurately, then verify the surrounding extension documentation remains consistent.

Written by the indexing model from the issue text.