Update clone docs to disuade concurrent Clone.
diff --git a/btree.go b/btree.go
index ea685a5..9b29c38 100644
--- a/btree.go
+++ b/btree.go
@@ -594,8 +594,9 @@
freelist *FreeList
}
-// Clone clones the btree, lazily. b2 can be used concurrently with
-// with the original tree, including concurrent writes to b and b2.
+// Clone clones the btree, lazily. Clone should not be called concurrently,
+// but the original tree (b) and the new tree (b2) can be used concurrently
+// once the Clone call completes.
//
// The internal tree structure of b is marked read-only and shared between b and
// b2. Writes to both b and b2 use copy-on-write logic, creating new nodes
