(Accepting Output): accept-process-output

uses microseconds, not milliseconds.  But that arg is obsolete.

lispref/processes.texi

@@ -1296,24 +1296,24 @@ or terminal input.  Occasionally it is useful in a Lisp program to
 explicitly permit output to arrive at a specific point, or even to wait
 until output arrives from a process.
 
-@defun accept-process-output &optional process seconds millisec just-this-one
+@defun accept-process-output &optional process seconds microsec just-this-one
 This function allows Emacs to read pending output from processes.  The
 output is inserted in the associated buffers or given to their filter
 functions.  If @var{process} is non-@code{nil} then this function does
 not return until some output has been received from @var{process}.
 
 @c Emacs 19 feature
-The arguments @var{seconds} and @var{millisec} let you specify timeout
+The arguments @var{seconds} and @var{microsec} let you specify timeout
 periods.  The former specifies a period measured in seconds and the
-latter specifies one measured in milliseconds.  The two time periods
+latter specifies one measured in microseconds.  The two time periods
 thus specified are added together, and @code{accept-process-output}
-returns after that much time whether or not there has been any
+returns after that much time, whether or not there has been any
 subprocess output.
 
-The argument @var{seconds} need not be an integer.  If it is a floating
-point number, this function waits for a fractional number of seconds.
-If @var{seconds} is 0, the function accepts whatever output is
-pending but does not wait.
+The argument @var{microsec} is semi-obsolete nowadays because
+@var{seconds} can be a floating point number to specify waiting a
+fractional number of seconds.  If @var{seconds} is 0, the function
+accepts whatever output is pending but does not wait.
 
 @c Emacs 22.1 feature
 If @var{process} is a process, and the argument @var{just-this-one} is