docs: clarify coordinate parameters for MatRipple.launch method. (#22806)

It was never clearly specified whether the coordinates for
`MatRipple.launch` should be relative to the trigger, ripple
container or viewport, or even absolute. This commit clarifies
this by explicitly mentioning the viewport.

Fixes #16613.

(cherry picked from commit 3462c22)

Author: devversion

src/material/core/ripple/ripple.ts

@@ -189,9 +189,11 @@ export class MatRipple implements OnInit, OnDestroy, RippleTarget {
   launch(config: RippleConfig): RippleRef;
 
   /**
-   * Launches a manual ripple at the specified coordinates within the element.
-   * @param x Coordinate within the element, along the X axis at which to fade-in the ripple.
-   * @param y Coordinate within the element, along the Y axis at which to fade-in the ripple.
+   * Launches a manual ripple at the specified coordinates relative to the viewport.
+   * @param x Coordinate along the X axis at which to fade-in the ripple. Coordinate
+   *   should be relative to the viewport.
+   * @param y Coordinate along the Y axis at which to fade-in the ripple. Coordinate
+   *   should be relative to the viewport.
    * @param config Optional ripple configuration for the manual ripple.
    */
   launch(x: number, y: number, config?: RippleConfig): RippleRef;