The signal is a key binding signal which gets emitted when the user requests a completion, by pressing <keycombo><keycap> Control</keycap><keycap>space</keycap></keycombo>.
This will create a [class@CompletionContext] with the activation type as gtk_source_completion_activation_user_requested.
Applications should not connect to it, but may emit it with [[email protected]_emit_by_name] if they need to activate the completion by another means, for example with another key binding or a menu entry.