Documented that prompters needn't be safe for concurrent usage.
Documented that prompters needn't be safe for concurrent usage.

This commit clarifies the behavioral requirements of prompters.
Specifically, it clarifies that they do not need to be safe for
concurrent usage.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen