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]

scss/_variables.scss

@@ -69,6 +69,7 @@ $enable-print-styles:       true !default;
 //
 // Control the default styling of most Bootstrap elements by modifying these
 // variables. Mostly focused on spacing.
+// You can add more entries to the $spacers map, should you need more variation.
 
 $spacer:   1rem !default;
 $spacer-x: $spacer !default;