Document SmartLock enabled by default. (#288)

Author: SUPERCILEX

auth/src/main/java/com/firebase/ui/auth/AuthUI.java

@@ -409,6 +409,8 @@ public SignInIntentBuilder setProviders(@NonNull String... providers) {
 
         /**
          * Enables or disables the use of Smart Lock for Passwords in the sign in flow.
+         * 
+         * <p>SmartLock is enabled by default
          */
         public SignInIntentBuilder setIsSmartLockEnabled(boolean enabled) {
             mIsSmartLockEnabled = enabled;