Update gulp API doc with info on file base

Expand on what file base is and how it affects the relative path calculation. Fixes #699.
The reason I've shown callback here was because this was the real situation I had run into when `gulp.src` was dealing with string paths instead of glob. I can rework the example into a contrived one and pass a string path directly instead of a callback:

```
gulp.src('app/css/somefile.scss')
  .pipe(gulp.dest('build'));
```

Let me know if this is preferred.