working_copy: update stale comment on `LockedWorkingCopy`
The recent refactoring introducing `WorkgingCopy::start_mutation()`
(25d19e8a65b8) made this comment incorrect.
@@ -851,8 +851,8 @@ impl WorkingCopy {
}
}
-// A working copy that's locked on disk. The tree state has already been
-// updated.
+/// A working copy that's locked on disk. The lock is held until you call
+/// `finish()` or `discard()`.
pub struct LockedWorkingCopy<'a> {
wc: &'a mut WorkingCopy,
#[allow(dead_code)]
