[doc] clarification around the scheduler

Author: mistercrunch

docs/scheduler.rst

@@ -4,9 +4,20 @@ The Scheduler
 The Airflow scheduler monitors all tasks and all dags and schedules the
 task instances whose dependencies have been met. Behinds the scene, 
 it monitors a folder for all dag objects it may contain,
-and periodically inspects all tasks to see whether is can schedule the
+and periodically inspects all tasks to see whether it can schedule the
 next run.
 
+The scheduler starts an instance of the executor specified in the your
+``airflow.cfg``, if it happens to be the LocalExecutor, tasks will be
+executed as subprocesses, in the case of CeleryExecutor, tasks are 
+executed remotely.
+
+To start a scheduler, simply run the command:
+
+.. code:: bash
+
+    airflow scheduler
+
 Note that: 
 
 * It **won't parallelize** multiple instances of the same tasks, it always wait for the previous schedule to be done to move forward