Explain BoundMethod and function_id a bit more

toolchain/sem_ir/typed_insts.h

@@ -384,8 +384,10 @@ struct BoolLiteral {
   BoolValue value;
 };
 
-// A bound method, that combines a function with the value to use for its
-// `self` parameter, such as `object.MethodName`.
+// A bound method, that combines a function with the value to use for its `self`
+// parameter, such as `object.MethodName`. It is a callable thing like the
+// `FunctionType` it refers to through the type of the `function_id`
+// instruction, rather than a call to the method itself like `Call`.
 struct BoundMethod {
   static constexpr auto Kind = InstKind::BoundMethod.Define<Parse::NodeId>(
       {.ir_name = "bound_method",