fix(lib): add missing JSDoc to ReadonlySet and fix Set#size grammar

Closes #63480: Set#size JSDoc said "in Set" — added the missing article
("in the Set") to match the grammar used in Map#size and everywhere else
in the file.

Closes #63481: ReadonlySet.forEach, ReadonlySet.has, and ReadonlySet.size
had no JSDoc at all. Added comments mirroring the corresponding Set<T>
members (updating the callback type reference from Set<T> to ReadonlySet<T>
where applicable).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Zelys-DFKH

src/lib/es2015.collection.d.ts

@@ -94,7 +94,7 @@ interface Set<T> {
      */
     has(value: T): boolean;
     /**
-     * @returns the number of (unique) elements in Set.
+     * @returns the number of (unique) elements in the Set.
      */
     readonly size: number;
 }
@@ -106,8 +106,17 @@ interface SetConstructor {
 declare var Set: SetConstructor;
 
 interface ReadonlySet<T> {
+    /**
+     * Executes a provided function once per each value in the Set object, in insertion order.
+     */
     forEach(callbackfn: (value: T, value2: T, set: ReadonlySet<T>) => void, thisArg?: any): void;
+    /**
+     * @returns a boolean indicating whether an element with the specified value exists in the Set or not.
+     */
     has(value: T): boolean;
+    /**
+     * @returns the number of (unique) elements in the Set.
+     */
     readonly size: number;
 }