ACTION-786 Draft some wording on features of the delivery environment that are different for mobile

This action came out of the Sophia F2F and was raised under a discussion 
of the push environment. I suggested that a little explanatory appendix 
would be nice, for people who are not familiar with mobile and its quirks.

I've drawn a little picture to try to scope what one might want to say 
about this and would welcome some discussion of what one might wish to 
say, how one might wish to say it and so on. Apologies but I didn't 
spend much time making this picture look nice, if it turns out to be 
worthwhile I will bring all my artistic skills to bear on it (both of them).

Jo