perf hists browser: Inform how to reset the symbol filter

When in the hists browser, i.e. in 'perf report' or in 'perf top', it is
possible to press '/' and specify a substring to filter by symbol name.

Clarify how to remove a filter by making the prompt be:

   Please enter the name of symbol you want to see.
   To remove the filter later, press / + ENTER

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-vbq2b0kyufwy6p0ctkfswcoe@git.kernel.org


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