doc: add some words about HWLOC_API_VERSION vs HWLOC_VERSION

Signed-off-by: Brice Goglin <[email protected]>

Author: bgoglin

doc/hwloc.doxy

@@ -3436,6 +3436,11 @@ and <tt>hwloc_get_api_version()</tt> came only in 1.1.1.
 Also do not use the old cpuset API since it was deprecated and superseded
 by the bitmap API in 1.1, and later removed in 1.5.
 
+If you ever need to look at the library version instead of the API version,
+you may want to use HWLOC_VERSION instead.
+Two stable releases of the same series usually have the same ::HWLOC_API_VERSION
+even if their HWLOC_VERSION are different.
+
 
 
 \htmlonly

include/hwloc.h

@@ -87,6 +87,10 @@ extern "C" {
  *
  * Users may check for available features at build time using this number
  * (see \ref faq_upgrade).
+ *
+ * \note This should not be confused with HWLOC_VERSION, the library version.
+ * Two stable releases of the same series usually have the same ::HWLOC_API_VERSION
+ * even if their HWLOC_VERSION are different.
  */
 #define HWLOC_API_VERSION 0x00020100