(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 don't have an authoritative source, but in https://owl.english.purdue.edu/owl/owlprint/607/ under extended rules for commas it states:
`2. Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.`
 * `b. Common introductory phrases that should be followed by a comma include participial and infinitive phrases, absolute phrases, nonessential appositive phrases, and long prepositional phrases (over four words).`


It's possible it was a nitpick for teachers in grade school, but I didn't really find an authoritative source.