SAMLParser: Fix incorrect assertion & documentation in parseElement().

The parseElement()-function actually accepts a
SAML2_XML_md_EntityDescriptor object, not a DOMElement.

Thanks to François Kooman for reporting this bug and supplying a patch!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3228 44740490-163a-0410-bde0-09ae8108e29a