docs: Improve prefill documentation (#1628)

Co-authored-by: Jan Čurn <[email protected]>

Author: janbuchar

sources/platform/actors/development/actor_definition/input_schema/specification.md

@@ -132,13 +132,13 @@ Each field of your input is described under its key in the `inputSchema.properti
 
 Here is a rule of thumb for whether an input field should have a `prefill`, `default`, or be required:
 
-- **Prefill** - Use for fields that don't have a reasonable default. The provided value is prefilled for the user to show them an example of using the field and to make it easy to test the Actor (e.g., search keyword, start URLs). In other words, this field is only used in the user interface but does not affect the Actor functionality and API.
+- **Prefill** - Use for fields that don't have a reasonable default. The provided value is prefilled for the user to show them an example of using the field and to make it easy to test the Actor (e.g., search keyword, start URLs). In other words, this field is only used in the user interface but does not affect the Actor functionality and API. Note that if you add a new input option to your Actor, the Prefill value won't be used by existing integrations such as Actor tasks or API calls, but the Default will be if specified. This is useful for keeping backward compatibility when introducing a new flag or feature that you prefer new users to use.
 - **Required** - Use for fields that don't have a reasonable default and MUST be entered by the user (e.g., API token, password).
 - **Default** - Use for fields that MUST be set for the Actor run to some value, but where you don't need the user to change the default behavior (e.g., max pages to crawl, proxy settings).
   If the user omits the value when starting the Actor via any means (API, CLI, scheduler, or user interface), the platform automatically passes the Actor this default value.
 - **No particular setting** - Use for purely optional fields where it makes no sense to prefill any value (e.g., flags like debug mode or download files).
 
-In summary, you can use each option independently or use a combination of **Prefill + Required**, but the combinations **Prefill + Default** or **Default + Required** don't make sense to use.
+In summary, you can use each option independently or use a combination of **Prefill + Required** or **Prefill + Default**, but the combination of **Default + Required** doesn't make sense to use.
 
 
 ## Additional properties