Vim: use `setlocal`, not `set` in ftplugins

Thanks to @lencioni[0] for pointing out that I was doing this wrong.

Some comments in this post[1], suggest that you can use `set` in
ftplugins, but other comments contradict that. It seems likely that the
nay-sayers are right.

[0]: https://github.com/lencioni
[1]: http://www.derekwyatt.org/2009/11/01/changing-settings-options-based-on-filetype-in-vim

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Vim: clarify some comments

These comments used to make sense, until I reordered the lines in the
file. Clear them up.

Signed-off-by: Wincent Colaiuta <win@wincent.com>