Currently the wsrep_sst_rsync script is symlinked to wsrep_sst_rsync_wan, with the script checking which name it was called by, and adding the --whole-file when not seeing wan in the $0 command name variable.
This is sort of magic, and not documented beyond having a minimal man page just saying
I'd suggest to have a [sst] parameter like e.g. rsync_wan, and maybe also a generic rsync_options instead to be able to have all configuration settings in one place.
And of course: document it properly ...