docs: mention locator.nth is zero based (#12750)

Co-authored-by: Pavel Feldman <pavel.feldman@gmail.com>

docs/src/api/class-framelocator.md

@@ -128,7 +128,7 @@ The method finds an element matching the specified selector in the FrameLocator'
 ## method: FrameLocator.nth
 - returns: <[FrameLocator]>
 
-Returns locator to the n-th matching frame.
+Returns locator to the n-th matching frame. It's zero based, `nth(0)` selects the first frame.
 
 ### param: FrameLocator.nth.index
 - `index` <[int]>

docs/src/api/class-locator.md

@@ -550,7 +550,7 @@ The method finds an element matching the specified selector in the `Locator`'s s
 ## method: Locator.nth
 - returns: <[Locator]>
 
-Returns locator to the n-th matching element.
+Returns locator to the n-th matching element. It's zero based, `nth(0)` selects the first element.
 
 ### param: Locator.nth.index
 - `index` <[int]>

packages/playwright-core/types/types.d.ts

@@ -9403,7 +9403,7 @@ export interface Locator {
   }): Locator;
 
   /**
-   * Returns locator to the n-th matching element.
+   * Returns locator to the n-th matching element. It's zero based, `nth(0)` selects the first element.
    * @param index
    */
   nth(index: number): Locator;
@@ -13846,7 +13846,7 @@ export interface FrameLocator {
   }): Locator;
 
   /**
-   * Returns locator to the n-th matching frame.
+   * Returns locator to the n-th matching frame. It's zero based, `nth(0)` selects the first frame.
    * @param index
    */
   nth(index: number): FrameLocator;