doc: Prepare for conversion to Doxygen

Fix many params in internal functions (not really necessary but Doxygen
warns about that in XML mode).

Fix formatting in a few corner cases that automatic conversion can't
handle.

Rearrange some DOC_DISABLE blocks.

include/libxml/parser.h

@@ -10,7 +10,6 @@
 #ifndef __XML_PARSER_H__
 #define __XML_PARSER_H__
 
-/** DOC_DISABLE */
 #include <libxml/xmlversion.h>
 #define XML_TREE_INTERNALS
 #include <libxml/tree.h>
@@ -27,7 +26,6 @@
 /* for compatibility */
 #include <libxml/SAX2.h>
 #include <libxml/threads.h>
-/** DOC_ENABLE */
 
 #ifdef __cplusplus
 extern "C" {