doc/go_mem: explain arrays, structs, and complex numbers

doc/go_mem: explain arrays, structs, and complex numbers

Arrays, structs, and complex numbers are collections of values that
are handled separately by the memory model.

An earlier version may have said this, but the current version does not.
Say it.

Change-Id: If3928bed6659e58e688f88aa0dde05423cbb3820
Reviewed-on: https://go-review.googlesource.com/c/go/+/514476
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>