Arvados

There's nothing in our documentation that actually tells you how to use arv-mount --exec. Some of arv-mount --help sort of hints at it, and you can figure it out if you're very familiar with the underlying conventions, but we can do better.

• Consider writing our own usage line(s) for arv-mount, rather than relying on the one generated by argparse. There are so many options that the generated one is unhelpfully long anyway. One should illustrate how to use arv-mount --exec. Others can illustrate other common operation modes.
• The help string for --exec should specify:
  • It reads the command as multiple arguments, until it reaches the end of arguments or --.
  • The command is not run through the shell. (This is important because it means your command can't use shell constructs, globs, etc.)
• If possible without writing new code, write a metavar for --exec that better illustrates that multiple arguments are used. There was a previous attempt at this (metavar is a tuple) but it doesn't work as intended, it just gets rendered as ....
• There should be no need for epilog text after these changes. We shouldn't expect users to read documentation in two places to learn how to use one feature.
• Add a section to doc/user/tutorials/tutorial-keep-mount-gnu-linux.html that illustrates how to use arv-mount --exec with a basic example.