API doc: fix and refine 'options.base' doc

[First example](https://github.com/gulpjs/gulp/blame/22979d1bb073a36da2b931deba1baa41e274ed47/docs/API.md#L55-L59) of `options.base` usage is wrong:

> ```js
// without the 'base' set
gulp.src('app/**/*.scss')        //          file: 'app/css/somefile.scss'
  .pipe(gulp.dest('build'));     //->   file base: 'app/css'
                                 // relative path: 'somefile.scss'
                                 //    write path: 'build/somefile.scss'
```

 `base` would resolve to `app` instead of `app/css`.

This PR fixes that glitch and attempts to clarify and make the `options.base` section more concise.