Improve explanation of "pad" option in phys2bids' help

[smoia]

Current Behavior

  -pad PAD, --padding PAD
                        Padding in seconds used around a single run when separating multi-run session files. Default is 9 seconds.

Better Behavior

Better to explain the option at more length: pad is the amount of seconds of recording that is saved around the real take, both before and after.
At the moment it should be less than the time between two takes (see issue )

Possible solution

Change description of pad in the argparse portion of cli.py