pthread: Fix pthread_cond_timedwait returning early from timeout

The reason timeouts would sometimes happen before the abstime deadline was due
to rounding errors converting the timeout to milliseconds, and also because
vTaskDelay(1) only delays until the next tick which is less than one full tick
period.

Closes https://github.com/espressif/esp-idf/issues/6901

docs/en/api-reference/system/pthread.rst

@@ -94,6 +94,8 @@ Condition Variables
 
 Static initializer constant ``PTHREAD_COND_INITIALIZER`` is supported.
 
+* The resolution of ``pthread_cond_timedwait()`` timeouts is the RTOS tick period (see :ref:`CONFIG_FREERTOS_HZ`). Timeouts may be delayed up to one tick period after the requested timeout.
+
 .. note:: These functions can be called from tasks created using either pthread or FreeRTOS APIs
 
 Thread-Specific Data