[security-services] Comment on core-25 sec 2.4.3.2

Section 2.4.3.2, the description of the AuthorityBinding element
(proposed by Simon G):

The element itself is optional, but within it, both attributes are
required (as in the schema), so delete the [Optional] or replace with
[Required] on lines 681 and 683.

Also I'd propose replacing the description in lines 679-80 with:

"The <AuthorityBinding> element specifies a SAML authority which can
provide additional assertions about the subject, identifies the type of
that authority, and points to its location (via a URI)."

(Q: Should the description specify that the authority will respond to
the SAML SOAP binding, or should it be left unspecified and
application-dependent?)

-- Scott