normalize help text for output of zed commands

[mccanne]

The various zed commands all support outputting data in any desired format.

The command help should describe this output uniformly, e..g, having some help text blurb that refers to the command outputting (not "printing") in any desired format. This work would involve coming up with the generic text and updating all of the command help with the new verbage.

This came up in PR #4984 with respect to the help text describing the process of outputting the VNG metadata in the new command zed dev vng.