runtime: fix documented alignment of 32KiB and 64KiB size classes

As Cherry pointed out on golang.org/cl/299909, the page allocator
doesn't guarantee any alignment for multi-page allocations, so object
alignments are thus implicitly capped at page alignment.

Change-Id: I6f5df27f269b095cde54056f876fe4240f69c5c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/301292


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>