[tasks] Fix sleep handling behaviour depending on interval type

Relative time intervals can be thought of as:

  for _ in range(count):
    await body()
    await asyncio.sleep(interval)

While explicit time intervals should be thought of as:

  times = [1pm, 2pm, 3pm, 12am]
  current = 0
  for _ in range(count):
    time = times.wrapping_index(current)  # magic to wrap around
    await utils.sleep_until(time)
    await body()
    current += 1

discord/ext/tasks/__init__.py

@@ -160,9 +160,14 @@ class Loop(Generic[LF]):
             self._next_iteration = self._get_next_sleep_time()
         else:
             self._next_iteration = datetime.datetime.now(datetime.timezone.utc)
+            await asyncio.sleep(0)  # allows canceling in before_loop
         try:
-            await self._try_sleep_until(self._next_iteration)
+            if self._stop_next_iteration:  # allow calling stop() before first iteration
+                return
             while True:
+                # sleep before the body of the task for explicit time intervals
+                if self._time is not MISSING:
+                    await self._try_sleep_until(self._next_iteration)
                 if not self._last_iteration_failed:
                     self._last_iteration = self._next_iteration
                     self._next_iteration = self._get_next_sleep_time()
@@ -178,6 +183,8 @@ class Loop(Generic[LF]):
                     if self._stop_next_iteration:
                         return
 
+                    # sleep after the body of the task for relative time intervals
+                    if self._time is MISSING:
                         await self._try_sleep_until(self._next_iteration)
 
                     self._current_loop += 1
@@ -346,6 +353,9 @@ class Loop(Generic[LF]):
             before stopping via :meth:`clear_exception_types` or
             use :meth:`cancel` instead.
 
+        .. versionchanged:: 2.0
+            Calling this method in :meth:`before_loop` will stop the first iteration from running.
+
         .. versionadded:: 1.2
         """
         if self._task is not MISSING and not self._task.done():
@@ -473,6 +483,9 @@ class Loop(Generic[LF]):
 
         The coroutine must take no arguments (except ``self`` in a class context).
 
+        .. versionchanged:: 2.0
+            Calling :meth:`stop` in this coroutine will stop the initial iteration from running.
+
         Parameters
         ------------
         coro: :ref:`coroutine <coroutine>`