add translation comments (closes #3962)

src/actions/advancement.cpp

@@ -219,6 +219,9 @@ namespace
 		}
 		virtual std::string description() const
 		{
+			// TRANSLATORS: In networked games, when one player has the choice
+			// between multiple advancements of a unit, this text is sent to
+			// other players. It will be embedded within a message.
 			return _("waiting for^an advancement choice");
 		}
 	private:

src/persist_var.cpp

@@ -51,6 +51,9 @@ struct persist_choice: mp_sync::user_choice {
 
 	virtual std::string description() const
 	{
+		// TRANSLATORS:  In networked games, this text is shown for other
+		// clients, while they wait to receive the content of a global variable
+		// from another player. This text will be embedded into a sentence.
 		return _("waiting for^a global variable");
 	}
 	virtual bool is_visible() const { return false; }

src/synced_user_choice.cpp

@@ -323,6 +323,9 @@ void user_choice_manager::update_local_choice()
 		}
 	}
 
+	// TRANSLATORS: In networked games, this text is shown on the map while
+	// waiting for $desc from another player.
+	// Don't end the text with a punctuation sign.
 	wait_message_ = VNGETTEXT(
 		"waiting for $desc from side $sides",
 		"waiting for $desc from sides $sides",

src/synced_user_choice.hpp

@@ -35,6 +35,9 @@ struct user_choice
 	///whether the choice is visible for the user like an advancement choice
 	///a non-visible choice is for example get_global_variable
 	virtual bool is_visible() const { return true; }
+	// TRANSLATORS: In networked games, this text is shown for other clients,
+	// while they wait for an action from another player.
+	// This text will be embedded into a sentence.
 	virtual std::string description() const { return _("waiting for^input"); }
 };