vty: absorb command explanations from osmo-gsm-manuals

The osmo-gsm-manuals/vty/sgsn_vty_additions.xml contained many command
explanations that are redundant with the VTY online doc. Some however are more
verbose / easier to understand. Absorb these into the online VTY doc here.

This matches the removal in osmo-gsm-manuals change-id
I71863e5056ad369d2055e9882a52a00fa999ab04.

Change-Id: I35984014424412e91437b7ed71576aef3819cb1e
1 file changed