hwloc/diff.h: improve the documentation about return values

Refs #578

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

include/hwloc/diff.h

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2013-2020 Inria.  All rights reserved.
+ * Copyright © 2013-2023 Inria.  All rights reserved.
  * See COPYING in top-level directory.
  */
 
@@ -222,6 +222,8 @@ enum hwloc_topology_diff_apply_flags_e {
 HWLOC_DECLSPEC int hwloc_topology_diff_apply(hwloc_topology_t topology, hwloc_topology_diff_t diff, unsigned long flags);
 
 /** \brief Destroy a list of topology differences.
+ *
+ * \return 0.
  */
 HWLOC_DECLSPEC int hwloc_topology_diff_destroy(hwloc_topology_diff_t diff);
 
@@ -233,6 +235,8 @@ HWLOC_DECLSPEC int hwloc_topology_diff_destroy(hwloc_topology_diff_t diff);
  * This identifier is usually the name of the other XML file
  * that contains the reference topology.
  *
+ * \return 0 on success, -1 on error.
+ *
  * \note the pointer returned in refname should later be freed
  * by the caller.
  */
@@ -246,6 +250,8 @@ HWLOC_DECLSPEC int hwloc_topology_diff_load_xml(const char *xmlpath, hwloc_topol
  * This identifier is usually the name of the other XML file
  * that contains the reference topology.
  * This attribute is given back when reading the diff from XML.
+ *
+ * \return 0 on success, -1 on error.
  */
 HWLOC_DECLSPEC int hwloc_topology_diff_export_xml(hwloc_topology_diff_t diff, const char *refname, const char *xmlpath);
 
@@ -257,6 +263,8 @@ HWLOC_DECLSPEC int hwloc_topology_diff_export_xml(hwloc_topology_diff_t diff, co
  * This identifier is usually the name of the other XML file
  * that contains the reference topology.
  *
+ * \return 0 on success, -1 on error.
+ *
  * \note the pointer returned in refname should later be freed
  * by the caller.
   */
@@ -274,6 +282,8 @@ HWLOC_DECLSPEC int hwloc_topology_diff_load_xmlbuffer(const char *xmlbuffer, int
  * The returned buffer ends with a \0 that is included in the returned
  * length.
  *
+ * \return 0 on success, -1 on error.
+ *
  * \note The XML buffer should later be freed with hwloc_free_xmlbuffer().
  */
 HWLOC_DECLSPEC int hwloc_topology_diff_export_xmlbuffer(hwloc_topology_diff_t diff, const char *refname, char **xmlbuffer, int *buflen);