Its name is Armory ingame but in configs its labeled “Library”. Inside the “cfgVehicles” where you define you vehicle:

class Library
{
libTextDesc = $STR_RHS_LIB_Hind;
};

and then in stringtable you write the description:

STR_RHS_LIB_Hind,“The Mil Mi-24 is a large combat helicopter gunship and low-capacity troop transport operated from 1976 by the Soviet Air Force”.

You can use it on CfgWeapons also, here is more comprehensive doc about it by DnA in BIS forums:

class Library 
{
  libEnabled: 1 (default, the item is listed) / 0 (the item is not listed)
  libTextDesc: string (containing the description text)
  type: 0 up to and including 11 (type override, allowing you to force the item into a certain category. Use of this entry should preferably be very limited.)
};