perf/scripts: Tag syscall_name helper as not yet available

syscall_name() helper, which resolves a syscall arch number to
its name, is not yet available as we first need to implement
event injection for it to work.

Remove it from the documentation or tag its references as
unavailable yet. Once it's implemented, we can just revert
the current patch.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Keiichi KII <k-keiichi@bx.jp.nec.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>