minor enhancement to prevent comment with unreferenced variable. edited

* genChRanges.py, chvalid.c, include/libxml/chvalid.h: minor
  enhancement to prevent comment with unreferenced variable.
* threads.c xmlreader.c xmlwriter.c: edited some comments to
  improve auto-generation of documentation
* apibuild.py: minor change to an error message
diff --git a/xmlreader.c b/xmlreader.c
index ec36fe9..d5dd377 100644
--- a/xmlreader.c
+++ b/xmlreader.c
@@ -1725,7 +1725,7 @@
 }
 
 /**
- * xmlTextReaderNextTree:
+ * xmlTextReaderNextSibling:
  * @reader:  the xmlTextReaderPtr used
  *
  * Skip to the node following the current one in document order while