document an implied invariant

Author: mikehaverstock

src/shared_types.d.ts

@@ -50,6 +50,11 @@ type LatexContext = {
     endIndex: number;
     selectionL?: NodeBase;
     selectionR?: NodeBase;
+
+    // in the end we expect to have either a cursorL or cursorParent
+    // to use to restore selection. We collect both because of shenanigans
+    // with MathBlock, and maybe others. If the cursorL is defined we will use it. Otherwise
+    // we will fall back to the cursorParent.
     cursorL?: NodeBase | 0;
     cursorParent?: NodeBase | 0;
   };