[pirc/new] add documentation to pir.y + add function documentation + add some comments here and there + add doc. about impl of constant folding and instr. select.
[pirc/new] add documentation to pir.y
+ add function documentation
+ add some comments here and there
+ add doc. about impl of constant folding and instr. select.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@30507 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

tewk/parrot-select,gitster/parrot,tkob/parrot,youprofit/parrot,gitster/parrot,FROGGS/parrot,tewk/parrot-select,youprofit/parrot,FROGGS/parrot,gagern/parrot,gagern/parrot,youprofit/parrot,tkob/parrot,parrot/parrot,FROGGS/parrot,fernandobrito/parrot,gagern/parrot,FROGGS/parrot,tkob/parrot,gitster/parrot,fernandobrito/parrot,tewk/parrot-select,fernandobrito/parrot,gitster/parrot,fernandobrito/parrot,parrot/parrot,tewk/parrot-select,gagern/parrot,FROGGS/parrot,youprofit/parrot,youprofit/parrot,fernandobrito/parrot,parrot/parrot,youprofit/parrot,gagern/parrot,parrot/parrot,parrot/parrot,youprofit/parrot,youprofit/parrot,tkob/parrot,tewk/parrot-select,FROGGS/parrot,fernandobrito/parrot,gagern/parrot,tkob/parrot,FROGGS/parrot,gagern/parrot,tkob/parrot,fernandobrito/parrot,gitster/parrot,tewk/parrot-select,gitster/parrot,gitster/parrot,tkob/parrot,tewk/parrot-select,tkob/parrot,FROGGS/parrot