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/xmlwriter.c b/xmlwriter.c
index 21897ee..38ee555 100644
--- a/xmlwriter.c
+++ b/xmlwriter.c
@@ -2565,7 +2565,7 @@
  * @name:  the name of the DTD
  * @pubid:  the public identifier, which is an alternative to the system identifier
  * @sysid:  the system identifier, which is the URI of the DTD
- * @format:  format string (see printf)
+ * @subset:  string content of the DTD
  *
  * Write a DTD.
  *