docs: Add a comment to the first example of Keyof Type Operator for beginners

Author: mymactive

packages/documentation/copy/en/handbook-v2/Type Manipulation/Keyof Type Operator.md

@@ -7,7 +7,8 @@ oneline: "Using the keyof operator in type contexts."
 
 ## The `keyof` type operator
 
-The `keyof` operator takes an object type and produces a string or numeric literal union of its keys:
+The `keyof` operator takes an object type and produces a string or numeric literal union of its keys.
+The following type P is the same type as "x" | "y":
 
 ```ts twoslash
 type Point = { x: number; y: number };