PIDF - examples have more places for extension than text

The examples in 4.3.2 and 4.3.3 have extension elements as direct children
of presence.  It doesn't say it can be there in the text.  I think it would
be good to allow extensions there as well as in the other 2 places it says
extensions can be (status and tuple).  I think in all 3 cases the extensions
should follow the impp defined element children - so apps using this know
all the impp defined standard stuff is first inside any element.  That isn't
necessary, but seems cleaner.

Example 4.3.2 has a mismatch in the names of an tags so one of them needs to
change
       <myex:complexExtension impp:mustUnderstand='YES'>
         ...
       </myex:mandatoryExtension>


  [reminder: [email protected] for non-technical discussions, please]