Document custom command action (#2921)

Co-authored-by: Pokey Rule <[email protected]>

Author: AndreasArvidsson

src/public_api.py

@@ -26,7 +26,7 @@ def cursorless_create_destination(
 
 @mod.action_class
 class CommandActions:
-    def cursorless_custom_command(
+    def cursorless_x_custom_command(
         content: str,  # pyright: ignore [reportGeneralTypeIssues]
         arg1: Optional[Any] = None,
         arg2: Optional[Any] = None,