(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.
Again not authoritative but I found a few more resources:

 * http://classroom.synonym.com/use-comma-after-prepositional-phrase-beginning-sentence-4012.html
* http://englishplus.com/grammar/00000074.htm

And arguments the other way: 
 * http://www.margieholdscourt.com/counting-those-words-in-prepositional-phrases/