Merge pull request firebase#694 from morganchen12/auth-docs

add FIRUser behavior to documentation

Author: morganchen12

Firebase/Auth/Source/Public/FIRUser.h

@@ -56,7 +56,11 @@ typedef void (^FIRSendEmailVerificationCallback)(NSError *_Nullable error)
     NS_SWIFT_NAME(SendEmailVerificationCallback);
 
 /** @class FIRUser
-    @brief Represents a user.
+    @brief Represents a user. Firebase Auth does not attempt to validate users
+        when loading them from the keychain. Invalidated users (such as those
+        whose passwords have been changed on another client) are automatically
+        logged out when an auth-dependent operation is attempted or when the
+        ID token is automatically refreshed.
     @remarks This class is thread-safe.
  */
 NS_SWIFT_NAME(User)