📝 Better description of Pick mapped type

closes https://github.com/Microsoft/TypeScript/issues/25976

src/lib/es5.d.ts

@@ -1410,7 +1410,7 @@ type Readonly<T> = {
 };
 
 /**
- * From T pick a set of properties K
+ * From T, pick a set of properties whose keys are in the union K
  */
 type Pick<T, K extends keyof T> = {
     [P in K]: T[P];