I found some documentation here:
http://luci.subsignal.org/trac/wiki/Documentation/CBI
So a textarea is called TextValue
:
o = s:option(TextValue, "_contact", "Kontakt")
o.default = string.format("%s", uci:get_first("gluon-contact", "contact", "public_info", ""))
o.rmempty = false
o.datatype = "string"
o.description = "z.B. E-Mail, Telefon oder Chat-Name"
o.size = 35