clarification to description of sender-vouches example

[Ron Monzillo <Ronald.Monzillo@Sun.COM>]

During the STP interop, we received 2 comments
that suggested that section 3.4.2.3 of the STP needed to
better describe the sender-vouches example.

To address these comments, I'd like to replace the descriptive
text at the beginning of section 3.4.2.3 with the following text.

I'd like to make this change before the next teleconf.
Comments and suggestions welcome.

Ron
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3.4.2.3 Example

The following example illustrates an attesting entity’s use of the 
sender-vouches subject confirmation method with an associated 
<ds:Signature> element to establish its identity and to assert that it 
has sent the message body on behalf of the subject(s) of the assertion 
referenced by “STR1”.

The assertion referenced by “STR1” is not included in the message. 
“STR1” is referenced by <ds:reference> from <ds:SignedInfo>. The 
ds:reference> includes the STR-transform to cause the assertion, not the 
<SecurityTokeReference> to be included in the digest calculation.

The SAML assertion embedded in the header and referenced by “STR2” from 
<ds:KeyInfo> corresponds to the attesting entity. The private key 
corresponding to the public confirmation key occurring in the assertion 
is used to sign together the message body and assertion referenced by 
“STRI”.