docs: add tooltip for Project category picker

Todos/View/ProjectPanelView.swift

@@ -38,7 +38,7 @@ struct ProjectPanelView: View {
         ForEach(allGroups) { group in
           Text(group.name)
         }
-      }
+      }.help("Default category for new Tasks")
       Text("Created on \(item.timestamp, format: style)")
     }
     List {