Document some new features and improvements since systemtap 1.6.

Document some new features and improvements since systemtap 1.6.

task_backtrace removed, MAXBACKTRACE documented, new probe_type() function,
unwind and symbol data only processed when really necesessary, kernel
backtraces for non-pt_regs probe context (timer and tracepoints),
ubacktrace() from kernel probe points, s390x user space support, ppc,
s390x and arm dwarf unwinder support, arm uprobes port.