Minor changes to comments, etc. for improving documentation generation

* encoding.c, threads.c, include/libxml/HTMLparser.h,
  doc/libxml2-api.xml: Minor changes to comments, etc. for
  improving documentation generation
* doc/Makefile.am: further adjustment to auto-generation of
  win32/libxml2.def.src
diff --git a/threads.c b/threads.c
index 8e7a08c..8db1aac 100644
--- a/threads.c
+++ b/threads.c
@@ -546,10 +546,15 @@
 }
 
 /**
- * DllMain
+ * DllMain:
+ * @hinstDLL: handle to DLL instance
+ * @fdwReason: Reason code for entry
+ * @lpvReserved: generic pointer (depends upon reason code)
  *
  * Entry point for Windows library. It is being used to free thread-specific
  * storage.
+ *
+ * Returns TRUE always
  */
 #if defined(HAVE_WIN32_THREADS) && !defined(LIBXML_STATIC)
 BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)