DOC Add doc in CommandRegistry about when it is updated (#316)

Adds some info about when `CommandRegistry` is updated. Uses info
learned when debugging #314

Happy to change wording or move to a better place.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Draga Doncila Pop <17995243+DragaDoncila@users.noreply.github.com>
Co-authored-by: Nathan Clack <nclack@chanzuckerberg.com>

src/npe2/_command_registry.py

@@ -39,6 +39,13 @@ def resolve(self) -> Callable:
 
 
 class CommandRegistry:
+    """Registry of commands, updated on `PluginManager.activate`.
+
+    `PluginManager.activate` is only run on `CommandRegistry.get` (when we resolve the
+    callable object for a command). This means that commands from enabled plugins are
+    not added to `CommandRegistry` until a command from that plugin is executed.
+    """
+
     command_registered = Signal(str)
     command_unregistered = Signal(str)