(PUP-3373) Clarify docstring for mount options

The docstring for the `options` parameter of the mount Type did not clarify
that the parameter value should be a single string. Also noted that many
operating systems use a comma-delimited list.
Merge pull request #3167 from Sharpie/PUP-3373-clarify-mount-options

(PUP-3373) (doc) Clarify options parameter docs for mount