message: Further explain unused_patches.
diff: @@ -42,9 +42,10 @@ pub struct Resolve {
  /// forwards compatibility.
  metadata: Metadata,
  /// `[patch]` entries that did not match anything, preserved in
  - /// `Cargo.lock` as the `[[patch.unused]]` table array.
  - /// TODO: *Why* is this kept in `Cargo.lock`? Removing it doesn't seem to
  - /// affect anything.
  + /// `Cargo.lock` as the `[[patch.unused]]` table array. Tracking unused
  + /// patches helps prevent Cargo from being forced to re-update the
  + /// registry every time it runs, and keeps the resolve in a locked state
  + /// so it doesn't re-resolve the unused entries.
  unused_patches: Vec<PackageId>,
  /// A map from packages to a set of their public dependencies
  public_dependencies: HashMap<PackageId, HashSet<PackageId>>,
  