(doc) Clarify options parameter docs for mount

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.
I learned this years ago, looking up a source now... it could be that it is no longer taught (and/or) it was just subjective to where I lived.

Strunk and White does not appear to have this written in it. In fact, the examples are quite the opposite.