minor: as per code-styple, add invariant comment

Author: matklad

crates/text_edit/src/lib.rs

@@ -17,6 +17,7 @@ pub struct Indel {
 
 #[derive(Default, Debug, Clone)]
 pub struct TextEdit {
+    /// Invariant: disjoint and sorted by `delete`.
     indels: Vec<Indel>,
 }