updated the docs and rebuild releasing 2.6.21 removed

* NEWS elfgcchack.h testapi.c doc/*: updated the docs and rebuild
  releasing 2.6.21
* include/libxml/threads.h threads.c: removed xmlIsThreadsEnabled()
* threads.c include/libxml/threads.h xmllint.c: added the more
  generic xmlHasFeature() as suggested by Bjorn Reese, xmllint uses it.
Daniel
diff --git a/doc/APIchunk23.html b/doc/APIchunk23.html
index 8a59e1d..a9194f7 100644
--- a/doc/APIchunk23.html
+++ b/doc/APIchunk23.html
@@ -440,6 +440,7 @@
 <a href="html/libxml-nanohttp.html#xmlNanoHTTPOpenRedir">xmlNanoHTTPOpenRedir</a><br />
 <a href="html/libxml-nanohttp.html#xmlNanoHTTPReturnCode">xmlNanoHTTPReturnCode</a><br />
 </dd><dt>requested</dt><dd><a href="html/libxml-parser.html#xmlExternalEntityLoader">xmlExternalEntityLoader</a><br />
+<a href="html/libxml-parser.html#xmlHasFeature">xmlHasFeature</a><br />
 <a href="html/libxml-valid.html#xmlIsID">xmlIsID</a><br />
 <a href="html/libxml-xmlmemory.html#xmlMallocFunc">xmlMallocFunc</a><br />
 <a href="html/libxml-xmlmemory.html#xmlReallocFunc">xmlReallocFunc</a><br />