fixed ID deallocation problem based on patch from Steve Shepard fixes bug

* valid.c: fixed ID deallocation problem based on patch from
  Steve Shepard fixes bug #160893
* xmlmemory.c: improving comment.
* testapi.c: new test for xmlDictExists() is generated.
Daniel
diff --git a/xmlmemory.c b/xmlmemory.c
index 69de28d..d547e2c 100644
--- a/xmlmemory.c
+++ b/xmlmemory.c
@@ -863,7 +863,8 @@
 /**
  * xmlCleanupMemory:
  *
- * Free up all the memory associated with memorys
+ * Free up all the memory allocated by the library for its own
+ * use. This should not be called by user level code.
  */
 void
 xmlCleanupMemory(void) {