Apply suggestions from code review

Co-authored-by: Raphaël Proust <[email protected]>

Author: c-cube

src/direct/lwt_direct.mli

@@ -23,11 +23,13 @@ val run_in_the_background :
 
 val yield : unit -> unit
 (** Yield to the event loop.
-    Can only be used inside {!run} or {!run_in_the_background}. *)
+    calling [yield] outside of {!run} or {!run_in_the_background} will raise an exception,
+    crash your program, or otherwise cause errors. It is a programming error to do so. *)
 
 val await : 'a Lwt.t -> 'a
 (** [await prom] returns the result of [prom], or re-raises the
     exception with which [prom] failed if it failed.
     If [prom] is not resolved yet, [await prom] will suspend the
     current task and resume it when [prom] is resolved.
-    Can only be used inside {!run} or {!run_in_the_background}. *)
+    calling [yield] outside of {!run} or {!run_in_the_background} will raise an exception,
+    crash your program, or otherwise cause errors. It is a programming error to do so. *)