Deprecate unnecessary main wrapper mixins

We're not sure these are useful, so these will be removed in a future release, if you are using this please let us know.

src/objects/_main-wrapper.scss

@@ -20,6 +20,7 @@
 // </div>
 
 
+/// @deprecated Replace this mixin with more direct references to the [spacing mixins](https://design-system.service.gov.uk/styles/spacing/#spacing-on-custom-components).
 @mixin govuk-main-wrapper {
   // In IE11 the `main` element can be used, but is not recognized  –
   // meaning it's not defined in IE's default style sheet,
@@ -37,7 +38,9 @@
   }
 }
 
-// Use govuk-main-wrapper--l when you page does not have Breadcrumbs, phase banners or back links
+/// Use govuk-main-wrapper--l when you page does not have Breadcrumbs, phase banners or back links.
+///
+/// @deprecated Replace this mixin with more direct references to the [spacing mixins](https://design-system.service.gov.uk/styles/spacing/#spacing-on-custom-components).
 @mixin govuk-main-wrapper--l {
   @include govuk-responsive-padding(8, "top");
 }