util: More doc for the Gerrit bot, add padding time to query age

util: More doc for the Gerrit bot, add padding time to query age

The following changes were made:
- Improve the wording of comments in the Python files and of the
documentation in the README file.
- Add 10 seconds to the query age so that the bot wouldn't miss
any new changes that could be missed due to time difference between
the Gerrit server and the bot.

Change-Id: Ic75f9572653a248230a8b4b0bd360a8d22efd371
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38155
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>