local_working_copy: fix documentation of TreeState::snapshot()

Since commit e716fe7, the function no longer returns a tree id. That
commit updated the function documentation to match the new behavior,
but then the old documentation was restored in commit ef83f2b, perhaps
by a bad merge conflict resolution.

lib/src/local_working_copy.rs

@@ -897,7 +897,7 @@ impl TreeState {
     }
 
     /// Look for changes to the working copy. If there are any changes, create
-    /// a new tree from it and return it, and also update the dirstate on disk.
+    /// a new tree from it.
     #[instrument(skip_all)]
     pub fn snapshot(&mut self, options: &SnapshotOptions) -> Result<bool, SnapshotError> {
         let SnapshotOptions {