clarify on comparability & uniqueness of handles (#802)

Author: Snektron

api/glossary.asciidoc

@@ -291,7 +291,10 @@ GL share group ::
 
 Handle ::
     An opaque type that references an _object_ allocated by OpenCL.
-    Any operation on an _object_ occurs by reference to that objects handle.
+    Any operation on an _object_ occurs by reference to that object's handle.
+    Each object must have a unique handle value during the course of its
+    lifetime. Handle values may be, but are not required to be, re-used by
+    an implementation.
 
 Happens before ::
     An ordering relationship between operations that execute on multiple