perf report/top: Make 'e' visible in the help and make it toggle showing callchains

The 'e' and 'c' hotkeys were present for a long time, but not documented
in the help window, change 'e' to be a toggle so that it gets consistent
with other toggles like '+' and document it in the help window.

Keep 'c' as is for people used to it but don't document, as it is easier
to just use 'e' to show/hide all the callchains for a top level
histogram entry.

Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-pmyi5x34stlqmyu81rci94x9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>