doc: improvements to fs.markdown copy

A major reworking of the fs.markdown structure and text. Moved things around so that they are organized by function, included additional information so there is less of dependency on knowledge of the POSIX API.

Other changes:

• Documents .path on fs.ReadStream and fs.WriteStream per https://github.com/nodejs/node/pull/4368
• Include coverage of the file system encoding issue
• Example enhancements throughout

This one will be difficult to review, and I apologize for that. @nodejs/documentation @bnoordhuis