Fix Trac #1814 (staging interaction in Template Haskell and GHCi), and add comments

Fix Trac #1814 (staging interaction in Template Haskell and GHCi), and add comments

An Id bound by GHCi from a previous Stmt is Global but Internal, and
I'd forgotten that, leading to unnecessary restrictions when using TH
and GHCi together.

This patch fixes the problem and adds lots of explanatory comments (which
is where most of the extra lines come from).