doc: to use the default scheduler, STARPU_SCHED_CTX_POLICY_NAME must be followed by an empty string, not by NULL

doc/doxygen/chapters/330_scheduling_contexts.doxy

@@ -1,7 +1,7 @@
 /*
  * This file is part of the StarPU Handbook.
 //  * Copyright (C) 2009--2011  Universit@'e de Bordeaux
- * Copyright (C) 2010, 2011, 2012, 2013, 2016  CNRS
+ * Copyright (C) 2010, 2011, 2012, 2013, 2016, 2017  CNRS
  * Copyright (C) 2011, 2012 INRIA
  * Copyright (C) 2016 Uppsala University
  * See the file version.doxy for copying conditions.
@@ -84,7 +84,7 @@ contexts in this traditional way.
 
 To create a <b>context</b> with the default scheduler, that is either
 controlled through the environment variable <c>STARPU_SCHED</c> or the
-StarPU default scheduler, one can explicitly use the option <c>STARPU_SCHED_CTX_POLICY_NAME, NULL</c> as in the following example:
+StarPU default scheduler, one can explicitly use the option <c>STARPU_SCHED_CTX_POLICY_NAME, ""</c> as in the following example:
 
 \code{.c}
 /* the list of resources the context will manage */
@@ -92,7 +92,7 @@ int workerids[3] = {1, 3, 10};
 
 /* indicate the list of workers assigned to it, the number of workers,
 and use the default scheduling policy. */
-int id_ctx = starpu_sched_ctx_create(workerids, 3, "my_ctx", STARPU_SCHED_CTX_POLICY_NAME, NULL, 0);
+int id_ctx = starpu_sched_ctx_create(workerids, 3, "my_ctx", STARPU_SCHED_CTX_POLICY_NAME, "", 0);
 
 /* .... */
 \endcode

doc/doxygen/chapters/api/scheduling_contexts.doxy

@@ -56,7 +56,8 @@ The arguments following the name of the scheduling context can be of
 the following types:
 <ul>
 <li> ::STARPU_SCHED_CTX_POLICY_NAME, followed by the name of a
-predefined scheduling policy
+predefined scheduling policy. Use an empty string to create the
+context with the default scheduling policy.
 </li>
 <li> ::STARPU_SCHED_CTX_POLICY_STRUCT, followed by a pointer to a
 custom scheduling policy (struct starpu_sched_policy *)