Main/mount doc wording

The [docs](http://docs.puppetlabs.com/references/3.6.0/type.html#mount-attribute-dump) read funny. Slight fixes to the relevant property code. Not really tested, though.

Note how 38984362 apparently introduced duplicate code. This was probably a result of James applying a not-quite-fitting [patch](https://projects.puppetlabs.com/issues/2405) to his tree.