[rfa/doco] Srink the overlay diagram

[Jim Blandy]

Andrew Cagney <ac131313@cygnus.com> writes:
> One of the things giving overfull hboxes is JimB's overlay diagram.
> The attached patch srinks it a little.

One of the points of the original diagram was that the unmapped
overlays (the ones in the "larger address space") weren't all the same
size --- they were all just no larger than the overlay area.  Now
they're all the same size.

Aside from that, yes, it's fine.