accept-process-output with nil PROCESS argument doesn't exit early

[Dmitry Gutov <dgutov@yandex.ru>]

According to the docstring, I'd expect this expression to finish in 1 
second and return t:

(progn
   (start-process-shell-command "sleep" "*sleep-command*"
                                "sleep 1; echo abc")
   (accept-process-output nil 2 nil))

But it takes 2 seconds instead and returns nil.

This one, with the process explicitly passed in, behaves like expected:

(progn
   (start-process-shell-command "sleep" "*sleep-command*"
                                "sleep 1; echo abc")
   (accept-process-output (get-buffer-process "*sleep-command*") 2))

Any idea what I'm doing wrong in the first example?