Make depth-charging code and documentation match

Make depth-charging code and documentation match

Before 4.0.6, depth charges required no guns, one military, did damage
like shell fire from two guns, and used two shells.  Missions were not
quite consistent with that (bug).  4.0.6 changed depth charges to work
exactly like shell fire (but without updating documentation
accordingly): require guns and gun crew, non-zero firing range, scale
damage and ammunition use with guns.

Go back to the old model, but with damage like three guns, to avoid
changing the stock game's dd now (three gun damage for two shells).
Stock game's af changes from two gun damage for one shell, and nas
from four gun damage for two shells.

Factor out common depth-charging code into shp_dchrg().