proposal for addressing last item in 3987

This proposal addresses item c  of bug 
3987.[http://www.w3.org/Bugs/Public/show_bug.cgi?id=3987]

Description: Section 5.9 'Lifecycle of Assertions' [1] in the Guidelines
document claims to cover three topics:

a) Assertion extensibility,
b) Policy language extensibility and
c) Subject attachment extensibility.




Target: Guidelines 


Description: Section 5 of the Guidelines currently has a third bullet 
under "Versioning Policy Assertions" which identifies 
Subject attachment Extensibility 
PolicyAttachment and AppliesTo also have extensibility points.

The proposal is to do 3 things:

1) replace this bullet with  "Supporting New Policy Subjects"
2)  add a new section [text below]
3) consider adding some new "good practices"  or extensions to the 
proposed set, i.e, G15 "An assertion description should specify a policy 
subject"  might consider adding " If the policy subjects change over time, 
the assertion description should also be versioned to reflect this 
change."

5.3 Supporting New Policy Subjects: 
  
Section 2 identifies that it is a best practice for policy authors to 
define the set of policy subjects to which policy assertions can be 
attached.  Over time, new policy subjects may need to be defined.  When 
this occurs, a policy assertion author may update the list of policy 
subjects supported by an assertion. 
  
When the assertion's semantics do not change to invalidate any of the 
original policy subjects but new policy subjects need to be added, it may 
be possible to use the same assertion to designate the additional policy 
subjects without a namespace change.  For example, a policy assertion for 
a protocol that is originally designed for endpoint policy subject may add 
message policy subject to indicate finer granularity in the attachment 
provided that endpoint policy subject is also retained in its design. When 
new policy subjects are added it is incumbent on the authors to retain the 
semantic of the policy assertion.