doc: Stability 2 (stable) listed as "unstable" in fs docs
https://github.com/nodejs/io.js/blob/master/doc/api/documentation.markdown#stability-index indicates that Stability 2 = Stable but there are three places in the doc/fs.md
where Stability 2 is marked Unstable. Not sure if the doc about stability index needs clarification, or if the fs
doc needs to update those indices to different values, or ... ?