Fix comment on IExtendableMessage.GetOrRegisterExtension

csharp/src/Google.Protobuf/IExtendableMessage.cs

@@ -50,8 +50,8 @@ namespace Google.Protobuf
         /// </summary>
         RepeatedField<TValue> GetExtension<TValue>(RepeatedExtension<T, TValue> extension);
 
-        /// <summary>
-        /// Gets the value of the specified repeated extension, registering it if it isn't
+        /// <summary>
+        /// Gets the value of the specified repeated extension, registering it if it hasn't already been registered.
         /// </summary>
         RepeatedField<TValue> GetOrRegisterExtension<TValue>(RepeatedExtension<T, TValue> extension);
 
@@ -65,13 +65,13 @@ namespace Google.Protobuf
         /// </summary>
         bool HasExtension<TValue>(Extension<T, TValue> extension);
 
-        /// <summary>
-        /// Clears the value of the specified extension
+        /// <summary>
+        /// Clears the value of the specified extension
         /// </summary>
         void ClearExtension<TValue>(Extension<T, TValue> extension);
 
-        /// <summary>
-        /// Clears the value of the specified repeated extension
+        /// <summary>
+        /// Clears the value of the specified repeated extension
         /// </summary>
         void ClearExtension<TValue>(RepeatedExtension<T, TValue> extension);
     }