stack(9): dynamic allocation is not necessary

The man page said dynamic allocation was required, but struct stack
can be allocated in any way, including on the stack. Make this clear,
and explain how to initialize the struct.

While I'm here, stack_save does not require any lock.

Reviewed by: markj, Pau Amma <pauamma_gundo.com>
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34461