Add note for bxl dynamic actions impl function argument

Summary:
The first argument of the impl function for dynamic actions must be bxl_ctx. However, this requirement is not mentioned in the documentation.

This diff adds a note to clarify it.

Reviewed By: iguridi

Differential Revision: D68860764

fbshipit-source-id: 19b00fb3d3568ac7c7522bdf499dbe687bac4f55

docs/bxl/how_tos/how_to_run_actions_based_on_the_content_of_artifact.md

@@ -60,6 +60,8 @@ dynamic attributes can be found [here](../../../api/build/dynattrs)
 
 We need to declare all arguments defined in step3 and bxl context.
 
+**Note:** The name of the first argument in the impl function must be `bxl_ctx`.
+
 We can read the `file` content by `read_string()` or `read_json()`. Details can
 be found [here](../../../api/build/ArtifactValue/)