doc: use "high address/low address" instead of "top/bottom"

The current document uses the "top" and "bottom" when talking
about the address within a frame, which may easily lead to
misunderstandings. This patch directly uses "high address"
and "low address" to make the expression clearer.

Change-Id: I7469330bbdc158672d7f0314fe6680ebdd9ab79a
Reviewed-on: https://go-review.googlesource.com/c/go/+/339369


Trust: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>