Clarify backfill strategy docs

Document the behavior of each context backfill strategy and make the
context-auto precedence explicit in the public options comments.  Tighten
the README wording so it describes the currently supported Create activity
handling instead of implying support for every Activity type.

Assisted-by: Codex:gpt-5

Author: sij411

packages/backfill/README.md

@@ -64,7 +64,8 @@ collection, it is skipped by ID.
 
 By default, `backfill()` uses the `context-auto` strategy.  In this mode,
 collection items are treated as backfillable objects by default.  If an item is
-recognized as an Activity, `backfill()` extracts the activity's object instead.
+recognized as a supported `Create` activity, `backfill()` extracts the
+activity's object instead.
 
 To read only FEP-f228 activity collections, enable the `context-activities`
 strategy:

packages/backfill/src/types.ts

@@ -3,6 +3,14 @@ import type { Object as APObject } from "@fedify/vocab";
 /**
  * Backfill traversal strategy used to discover the returned object.
  *
+ * -  `"context-objects"` yields post-like objects directly from the context
+ *    collection.
+ * -  `"context-activities"` yields objects extracted from supported `Create`
+ *    activities in the context collection.
+ * -  `"context-auto"` classifies context collection items automatically,
+ *    handling direct post-like objects and supported `Create` activities.
+ *    If included, it absorbs all other strategies.
+ *
  * @since 2.x.0
  */
 export type BackfillStrategy =
@@ -63,6 +71,7 @@ export interface BackfillOptions<
    * Backfill strategies to run.
    *
    * Defaults to `["context-auto"]`.
+   * If `"context-auto"` is included, it absorbs all other strategies.
    *
    * @since 2.x.0
    */