Documentation for sum() function should indicate Python syntax *first*

[rlm]

When did we hijack the sum function? Based on the documentation there, I have (today alone) had four different people come up to me and ask why something like the following doesn't work:

sage: sum(Integer(x), x, 0, 9)

I know the reasons this shouldn't work, but newbies definitely don't. It should say something about how to do

sage: sum( Integer(x) for x in range(10) )

before "getting all symbolic."