Allow valid message invalidation with new default functions (#1025)

* Allow valid message invalidation with new default functions

Fixes (part of) #1024.

@macchiati Demonstrated that the stability policy could logically conflict with locally-assigned default-namespace functions or options. This fixes the policy to allow us to assign **_new_** values that conflict with such.

* Update spec/intro.md

* Update spec/intro.md

Co-authored-by: Eemeli Aro <[email protected]>

---------

Co-authored-by: Eemeli Aro <[email protected]>

Author: aphillips

spec/intro.md

@@ -78,7 +78,7 @@ A reference to a _term_ looks like this.
 
 ### Stability Policy
 
-Updates to this specification will not make any valid _message_ invalid.
+Updates to this specification will not make any _valid_ _message_ become not _valid_.
 
 Updates to this specification will not specify an _error_ for any _message_
 that previously did not specify an _error_.