luci-app-wireguard: add descriptions

Add description fields to luci-app-wireguard similar to those in
luci-proto-wireguard.

Ref: https://github.com/openwrt/luci/commit/140f41b4


Suggested-by: "rwalli" <rwalli@gmx.net>
[squash commits, reword commit message, remove semicolons,
 remove trailing white space]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>