perf report: Amend documentation about max_stack and synthesized callchains

The --max_stack option was added as an optimization to reduce processing time,
so people specifying --max-stack might get a increased processing time if
combined with synthesized callchains, but otherwise no real harm.

A warning about setting both --max_stack and the synthesized callchains max
depth seems like overkill.  Amend the documentation.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/560A5155.4060105@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>