tmpfiles: split out verbs in help

tmpfiles: split out verbs in help

One of the three must always be specified, but they buried in a long list of
options in the output of --help. Make them more visible to draw the eye.

Also, drop "marked" from the description. It's supposed to mean "configured",
but it's a strange way to say that, and also it's generally obvious that the
program does what its configuration tells it to, and it's not going to remove
all files found on the system.