PIR user documentation

[Allison Randal]

Allison Randal wrote:
> 
> docs/user/pir/pmcs.pod

On a generally related topic, for user docs (and academic papers) we're 
going to need a more meaningful expansion of PMC than "Parrot Magic 
Cookie". If "Perl" can be both "Practical Extraction and Report 
Language" and "Pathologically Eclectic Rubbish Lister", then PMC can 
certainly have a dual meaning. Possibly:

- "Parrot Mid-level Class" (between low-level pure C, and a high-level 
PIR class)

- "PolyMorphic Container"

Allison