21504: Rewrite arv-mount help for more consistent style

21504: Rewrite arv-mount help for more consistent style

Some key points:

* Prefer describing the effect with a phrase over a sentence
* Only use periods when there are multiple sentences
* Defaults in parentheses
* Consistent text where needed (particularly across --by-* and
  --mount-by-* options and different --unmount options)

We don't really have a style guide for documentation like this. I'm not
trying to establish one by fiat. I'd be open to discussing basically all
these points. But until that discussion happens, consistency is
valuable, so I'm using the rules that I tend to follow naturally.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>