clarify docs

csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs

@@ -133,6 +133,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>EnumOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs

@@ -78,6 +78,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>EnumValueOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs

@@ -309,6 +309,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>FieldOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs

@@ -552,6 +552,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>FileOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs

@@ -292,6 +292,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>MessageOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs

@@ -78,6 +78,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>MethodOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/OneofDescriptor.cs

@@ -122,6 +122,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>OneofOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>

csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs

@@ -99,6 +99,7 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// The <c>ServiceOptions</c>, defined in <c>descriptor.proto</c>.
+        /// If the options message is not present (=there are no options), <c>null</c> is returned.
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>