Document how MAP_STACK will be used. All stacks must be mmap'd with

Document how MAP_STACK will be used.  All stacks must be mmap'd with
this attribute.  The kernel does so for main-process stacks at execve() time,
pthread stack functions do so for new stacks, and stacks provided to
sigaltstack() and other user-provided stacks will need to be allocated
in that way.
Not required yet, but paving the way.
Work done with stefan