Regenerating docs and API files

Various cleanups
* configure.in: force regeneration of APIs in my environment
* buf.c buf.h enc.h encoding.c include/libxml/tree.h
  include/libxml/xmlerror.h save.h tree.c: various comment cleanups
  pointed by apibuild
* doc/apibuild.py: added the 3 new internal headers in the excludes
* doc/libxml2-api.xml doc/libxml2-refs.xml: regenerated the API
* doc/symbols.xml: listing new entry points for 2.9.0
* doc/devhelp/*: regenerated
diff --git a/doc/apibuild.py b/doc/apibuild.py
index c2dff84..c9d5173 100755
--- a/doc/apibuild.py
+++ b/doc/apibuild.py
@@ -52,7 +52,10 @@
   "test.c": "not part of the library",
   "testdso.c": "test for dynamid shared libraries",
   "testrecurse.c": "test for entities recursions",
-  "xzlib.h": "Internal API only",
+  "xzlib.h": "Internal API only 2.8.0",
+  "buf.h": "Internal API only 2.9.0",
+  "enc.h": "Internal API only 2.9.0",
+  "save.h": "Internal API only 2.9.0",
 }
 
 ignored_words = {