perf script: add -S/--symbols documentation

Capture both that this option exists and that symbols can be hexadecimal
addresses.

Signed-off-by: Ian Rogers <irogers@google.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200402174130.140319-1-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>