exp run: clarify --rev help output

[jorgeorpinel]

... the CLI output Continue the specified checkpoint experiment isn't as clear as the docs continue an experiment from a specific checkpoint.

You don't need --rev to resume a checkpoint experiment, you need it to specify from which checkpoint to resume from.

Originally posted by @jorgeorpinel in iterative/dvc.org#3319 (comment) Cc @omesser

• Notice the term should be "resume" instead of "continue", which probably applies to other related output strings.

[dberenbaum]

No longer an available option