docs: add a little note about avoiding 'vertical' where possible

Author: bradenmacdonald

src/generic/key-utils.ts

@@ -52,6 +52,14 @@ export const buildCollectionUsageKey = (learningContextKey: string, collectionId
 
 export enum ContainerType {
   Unit = 'unit',
+  /**
+   * Vertical is the old name for Unit. Generally, **please avoid using this term entirely in any libraries code** or
+   * anything based on the new Learning Core "Containers" framework - just call it a unit. We do still need to use this
+   * in the modulestore-based courseware, and currently the /xblock/ API used to copy library containers into courses
+   * also requires specifying this, though that should change to a better API that does the unit->vertical conversion
+   * automatically in the future.
+   * TODO: we should probably move this to a separate enum/mapping, and keep this for the new container types only.
+   */
   Vertical = 'vertical',
 }