Add links for history reasons in file.py upload voodoo

[kofalt]

@josschne @gsfr @nagem Ref this never-expected-to-be-responded-to PR that I filed on webapp:
moraes/webapp-improved#12. If this jives with our current understanding of our override situation, we should probably throw a link in the comments on the upload voodoo in files.py.

This is a ticket, rather than a tiny commit / PR, because I want to check with the team if a) my PR description is still correct as far as we know, b) what other links are useful for understanding our tempfile / memory form processing situation.

[coltonlw]

I added comments explaining why we do these tricky things in files.py including links to cgi and webob source