doc: withPhoneSignIn() callback

Author: russellwheatley

FirebaseSwiftUI/README.md

@@ -803,7 +803,7 @@ authService
 public func withPhoneSignIn() -> AuthService
 ```
 
-Enables phone number authentication with SMS verification.
+Enables phone number authentication with SMS verification. Uses default behavior (navigates to enter phone number view when tapped).
 
 **Example:**
 
@@ -812,6 +812,21 @@ authService
   .withPhoneSignIn()
 ```
 
+```swift
+public func withPhoneSignIn(onTap: @escaping () -> Void) -> AuthService
+```
+
+Enables phone authentication with a custom callback (i.e where to navigate when tapped) when the phone button is tapped.
+
+**Example:**
+
+```swift
+authService
+  .withPhoneSignIn(){
+    // navigate to phone sign-in screen logic
+  }
+```
+
 ---
 
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