Add catch up behavior to xTaskDelayUntil. (#1348)

Adds an explanation for why xTaskDelayUntil
can break from the configured frequency when
catching up for past events. More will be added
to the website documentation.

include/task.h

diff --git a/include/task.h b/include/task.h
index a25740e..8ecfeab 100644
--- a/include/task.h
+++ b/include/task.h
@@ -897,7 +897,8 @@
  *
  * @return Value which can be used to check whether the task was actually delayed.
  * Will be pdTRUE if the task way delayed and pdFALSE otherwise.  A task will not
- * be delayed if the next expected wake time is in the past.
+ * be delayed if the next expected wake time is in the past. This prevents periodic
+ * tasks from accumulating delays and allows them to resume their regular timing pattern.
  *
  * Example usage:
  * @code{c}