Add Eric's comments from the PR about __i and __basic_sender and

`__sexpr`.

include/stdexec/__detail/__basic_sender.hpp

@@ -500,8 +500,8 @@ namespace stdexec {
   using __get_attrs_fn =
     __result_of<__detail::__drop_front, __mtypeof<__sexpr_impl<_Tag>::get_attrs>>;
 
-  //////////////////////////////////////////////////////////////////////////////////////////////////
-  // __basic_sender
+  //! A dummy type used only for diagnostic purposes. 
+  //! See `__sexpr` for the implementation of P2300's _`basic-sender`_.
   template <class...>
   struct __basic_sender {
     // See MAINTAINERS.md#class-template-parameters for `__id` and `__t`.
@@ -510,7 +510,9 @@ namespace stdexec {
   };
 
   //! A struct template to aid in creating senders.
-  //! A `__sexpr<Desc>` 
+  //! This struct closely resembles P2300's [_`basic-sender`_](https://eel.is/c++draft/exec#snd.expos-24),
+  //! but is not an exact implementation.
+  //! Note: The struct named `__basic_sender` is just a dummy type and is also not _`basic-sender`_.
   template <auto _DescriptorFn, class = __anon>
   struct __sexpr {
     using sender_concept = sender_t;

include/stdexec/__detail/__meta.hpp

@@ -329,9 +329,15 @@ namespace stdexec {
   template <class... _Args>
   concept _Ok = (STDEXEC_IS_SAME(__ok_t<_Args>, __msuccess) && ...);
 
-  //! If both are true:
-  //! Then __i<true, true>::__g<F, Args...> is an alias for F<Args...>
-  //! and __i<true, true>::__f<F> is an alias for F. 
+  //! The struct `__i` is the implementation of P2300's  
+  //! [_`META-APPLY`_](https://eel.is/c++draft/exec#util.cmplsig-5).
+  //! > [Note [1](https://eel.is/c++draft/exec#util.cmplsig-note-1): 
+  //! > The purpose of META-APPLY is to make it valid to use non-variadic 
+  //! > templates as Variant and Tuple arguments to gather-signatures. — end note]
+  //! In addition to avoiding the dreaded "pack expanded into non-pack argument" error,
+  //! it is part of the meta-error propagation mechanism. if any of the argument types 
+  //! are a specialization of `_ERROR_`, `__i` will short-circuit and return the error.
+  //! `__minvoke` and `__meval` are implemented in terms of `__i`.
   template <bool _ArgsOK, bool _FnOK = true>
   struct __i;