swarming: improve the signal handling documentation
swarming: improve the signal handling documentation

Otherwise it's not documented anywhere that it works, so nobody will ever use
it.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org
Bug:764385
Change-Id: I3e016950eb12b7e6c5d0faf01b187b523d2a2bf4
Reviewed-on: https://chromium-review.googlesource.com/673852
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py