runtime: clarify table of arena sizes

Currently the table of arena sizes mixes the number of entries in the
L1 with the size of the L2. While the size of the L2 is important,
this makes it hard to see what's actually going on because there's an
implicit factor of sys.PtrSize.

This changes the L2 column to say both the number of entries and the
size that results in. This should hopefully make the relations between
the columns of the table clearer, since they can now be plugged
directly into the given formula.

Change-Id: Ie677adaef763b893a2f620bd4fc3b8db314b3a1e
Reviewed-on: https://go-review.googlesource.com/c/139697


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>