runtime: improve mheap.alloc doc and let compiler check system stack

The alloc_m documentation refers to concepts that don't exist (and
maybe never did?). alloc_m is also not the API entry point to span
allocation.

Hence, rewrite the documentation for alloc and alloc_m. While we're
here, document why alloc_m must run on the system stack and replace
alloc_m's hand-implemented system stack check with a go:systemstack
annotation.

Change-Id: I30e263d8e53c2774a6614e1b44df5464838cef09
Reviewed-on: https://go-review.googlesource.com/c/139459


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>