adding notes to the flags for xshear_metacal.py

[hywu]

--runall # if not set, only the main file
# if set, runs the other metacal files (1p,1m,2p,2m), totally 5 times
--head # uses only random subset of source catalog
--nopairs # does not log source-lens pairs
--noclust # skip clusters
--norands # skip randoms
--params PARAMS # forces to use config file at path PARAMS
--metachunk METACHUNK # reruns the i-th (out of 5) metacal versions
--nchunk NCHUNK
--ichunk ICHUNK