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
  nodejs#4368
* Include coverage of the file system encoding issue
* Example enhancements throughout