On the mentioned default.c page you can find this, when you search for F11:
[F11] Activates a Statistics Panel (see below) that displays some predefined variables and other information. [F11] pressed a second time causes level and terrain polygon tesselation to be highlighted by red lines, and the bounding boxes of non-passable entities by a blue wireframe.
Unfortunately there is no any word for yellow lines or wireframes.
