Clarify thread switching while waiting for process output

* doc/lispref/threads.texi (Threads): Clarify, that thread
switching happens when waiting for process output from
asynchronous processes.

doc/lispref/threads.texi

@@ -17,9 +17,9 @@ correct programs should not rely on cooperative threading.
 
   Currently, thread switching will occur upon explicit request via
 @code{thread-yield}, when waiting for keyboard input or for process
-output (e.g., during @code{accept-process-output}), or during blocking
-operations relating to threads, such as mutex locking or
-@code{thread-join}.
+output from asynchronous processes (e.g., during
+@code{accept-process-output}), or during blocking operations relating
+to threads, such as mutex locking or @code{thread-join}.
 
   Emacs Lisp provides primitives to create and control threads, and
 also to create and control mutexes and condition variables, useful for