doc: clarify pos-list all-from-store attribute is optional

Author: jg10-mastodon-social

storybook/stories/6b_pos-list-if-typeof.stories.mdx

@@ -14,7 +14,8 @@ import { Canvas, Meta, Story } from "@storybook/addon-docs/blocks";
 ## pos-list if-typeof
 
 Renders all things of a given type found in the current store, with a custom
-template. The related resources can optionally be fetched before rendering.
+template. The related resources can optionally be fetched before rendering. The
+attribute `all-from-store` is optional and inferred by default.
 
 <Canvas withSource="open">
   <Story name="pos-list if-typeof">