perf top: Document --ignore-vmlinux

perf top: Document --ignore-vmlinux

We've had this since 2013, document it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Willy Tarreau <w@1wt.eu>
Fixes: 4bddc018b06d ("perf symbols: Add new option --ignore-vmlinux for perf top")
Link: https://lkml.kernel.org/n/tip-0jwfueooddwfsw9r603belxi@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>