Document static spacing override classes

Static spacing override classes are being introduced to GOV.UK Frontend (alphagov/govuk-frontend#2672).

Document the new classes so that users know how they work, and how they compare to the existing responsive spacing override classes.

src/styles/spacing/index.md.njk

@@ -179,11 +179,11 @@ margin-top: govuk-spacing(-3);
 
 ## Overriding spacing
 
-Occasionally, you might need to make minor adjustments like adding or removing spacing to elements of your design. You can use the responsive spacing override classes for this.
+Occasionally, you might need to make minor adjustments like adding or removing spacing to elements of your design. You can use the spacing override classes for this.
 
-### Using the override classes
+### Responsive spacing override classes
 
-The spacing override classes start with: `govuk-!-`, followed by either `margin-` or `padding-`, and then a spacing unit number.
+The responsive spacing override classes start with: `govuk-!-`, followed by either `margin-` or `padding-`, and then a spacing unit number.
 
 To apply spacing in a single direction, include `left-`, `right-`, `top-`, or `bottom-` just before the spacing unit.
 
@@ -193,6 +193,16 @@ For example:
 - `govuk-!-padding-right-5` will apply 15px of padding to the right side of the element on small screens, increasing to 25px on large screens
 - `govuk-!-margin-0` will remove all margins at all screen sizes
 
+### Static spacing override classes
+
+The static spacing override classes work the same way as the responsive spacing override classes, but start with `govuk-!-static`.
+
+For example:
+
+- `govuk-!-static-margin-9` will apply a 60px margin to all sides of the element at all screen sizes
+- `govuk-!-static-right-5` will apply 25px of padding to the right side of the element at all screen sizes
+- `govuk-!-static-margin-0` will remove all margins at all screen sizes, same as `govuk-!-margin-0`
+
 ### Examples
 
 {{ example({group: "styles", item: "spacing", example: "spacing-scale-padding", html: true, open: false, size: "l"}) }}