docs/library: Document that sleep(0)/sleep_ms(0) run the scheduler.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

docs/library/micropython.rst

@@ -130,6 +130,10 @@ Functions
      a critical region but they will not be executed until that region
      is exited.  An example of a critical region is a preempting interrupt
      handler (an IRQ).
+   - Inside native code functions, scheduled functions are not called unless
+     the native code calls a function that specifically does so.
+   - ``time.sleep`` and ``time.sleep_ms``, including zero-duration sleeps,
+     will call scheduled functions.
 
    A use for this function is to schedule a callback from a preempting IRQ.
    Such an IRQ puts restrictions on the code that runs in the IRQ (for example

docs/library/time.rst

@@ -71,6 +71,9 @@ Functions
    other boards may not accept a floating-point argument, for compatibility with
    them use `sleep_ms()` and `sleep_us()` functions.
 
+   Calling ``sleep``, including ``sleep(0)`` is guaranteed to call pending callback
+   functions.
+
 .. function:: sleep_ms(ms)
 
    Delay for given number of milliseconds, should be positive or 0.
@@ -80,6 +83,9 @@ Functions
    interrupt handlers or other threads.  Passing in 0 for *ms* will still allow
    this other processing to occur.  Use `sleep_us()` for more precise delays.
 
+   Calling ``sleep_ms``, including ``sleep_ms(0)`` is guaranteed to call
+   pending callback functions.
+
 .. function:: sleep_us(us)
 
    Delay for given number of microseconds, should be positive or 0.