Document the possibility of extending the $spacers map

Document the possibility of extending the $spacers map

It's very handy to add another spacer size (e.g. `4`) which can easily be done by extending the map.
Example:
```
$spacers: (
  0: (
    x:   0,
    y:   0
  ),
  1: (
    x:   $spacer-x,
    y:   $spacer-y
  ),
  2: (
    x:   ($spacer-x * 1.5),
    y:   ($spacer-y * 1.5)
  ),
  3: (
    x:   ($spacer-x * 3),
    y:   ($spacer-y * 3)
  ),
  4: (
    x:   ($spacer-x * 5),
    y:   ($spacer-y * 5)
  )
);
```

Closes #18706
[skip sauce]