add better docs

Author: morganchen12

FirebasePhoneAuthUI/Sources/Public/FirebasePhoneAuthUI/FUIPhoneAuth.h

@@ -30,6 +30,8 @@ NS_ASSUME_NONNULL_BEGIN
 
 /** @property defaultCountryCode
     Returns the default country code. If unspecified, this will default to the United States country code.
+    If overwriting the default country code, this value must be set before the phone auth flow is
+    presented.
 */
 @property (nonatomic, copy, readwrite) NSString *defaultCountryCode;