Apply VTY reference changes from libosmocore: change node IDs from index
numbers to meaningful names from VTY node prompts.
Introduce section with common commands, do not repeat the common commands on
each child node.
Populate section names (so far empty).
Add apn / gpdu VTY reference.
This is generated using the recent libosmocore vty doc patches that conclude in
libosmocore change-id Iedd67750539b676271de0e0e9316d4e6f794406a.
Change-Id: Ia269c4bda0aa0b905abcccc75338f5f808e01727