update breadcrumbs usage guidance

[indirectlylit]

In a recent change related to granular import/export (#2610), we added a new prop to the core breadcrumbs component:

+      /**
 +       * When set to 'true', a breadcrumb will be shown for each item in 'items' array.
 +       * Otherwise, the first item will be omitted.
 +       */
 +      showAllCrumbs: {
 +        type: Boolean,
 +        default: false,
 +      },

https://github.com/learningequality/kolibri/pull/2610/files#diff-d37333989ca3443ceed46359f1fab1feR113

We need to update the style guide to give guidance on when this prop is intended to be used.

cc @khangmach @jtamiace @jonboiser @christianmemije