Document rate limit and ETA incompatibilities (#3899)

Fixes #3888

docs/internals/worker.rst

@@ -38,6 +38,10 @@ When a message is received it's converted into a
 Tasks with an ETA, or rate-limit are entered into the `timer`,
 messages that can be immediately processed are sent to the execution pool.
 
+ETA and rate-limit are 2 incompatible parameters, and the ETA is overriding
+the rate-limit by default. A task with both will follow its ETA and ignore its
+rate-limit.
+
 Timer
 -----
 

docs/userguide/tasks.rst

@@ -856,6 +856,10 @@ General
     maximum number of  requests per second), you must restrict to a given
     queue.
 
+    .. note::
+
+        This attribute is ignored if the task is requested with an ETA.
+
 .. attribute:: Task.time_limit
 
     The hard time limit, in seconds, for this task.