Address review: explain why CborType | ReadonlyCborType union is needed

bartlomieju noted on #7148 that the union looks redundant at first
glance because mutable Map/array/index-signature are all assignable
to their readonly counterparts. The reason it is needed is that
CborTag<T> has a writable tagContent field and is therefore
invariant in T, so CborTag<CborType> is not assignable to
CborTag<ReadonlyCborType> and the union preserves existing CborTag
callers. Document that on ReadonlyCborType so the union is not
'simplified' away later.

Author: LeSingh1

cbor/types.ts

@@ -59,6 +59,13 @@ export type CborType =
  * encodeCbor(data);
  * ```
  */
+// Note: encoder signatures take `CborType | ReadonlyCborType` rather than
+// just `ReadonlyCborType`. Mutable `Map`/array/index-signature are all
+// assignable to their readonly counterparts, so the union looks redundant,
+// but `CborTag<T>` has a writable `tagContent: T` field, which makes it
+// invariant in `T`. That means `CborTag<CborType>` is NOT assignable to
+// `CborTag<ReadonlyCborType>`, and existing callers passing
+// `CborTag<CborType>` instances would break if the union were collapsed.
 export type ReadonlyCborType =
   | CborPrimitiveType
   | CborTag<ReadonlyCborType>