documentation fixes: ofFileUtils.h

Hey all, a few of these low level doc PRs got merged before I got to take a look at them -- for ofFileUtils.h a couple things --

To conform to doxygen style guide (https://github.com/openframeworks/openFrameworks/wiki/openFrameworks-doxygen-documentation-style-guidelines)

• Mixed use of \return \returns - both are valid, but style guide suggests \returns
• Need complete sentences with capitalization / punctuation, it's -> its where appropriate.
• 80 char hard breaks unless impossible.
• Need \brief line with a blank line after (for parsers -- \brief tag will be removed later) and order should
  be:
  • brief
  • blank
  • description
  • notes, warnings, throws, etc
  • params
  • returns

I'm happy to take a pass unless someone else wants to.

Thanks @danomatika for all of your work on these!