docs: Document Syscall User Dispatch

Explain the interface, provide some background and security notes.

[ tglx: Add note about non-visibility, add it to the index and fix the
  	kerneldoc warning ] 

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20201127193238.821364-8-krisman@collabora.com