jsdoc: print error on unknown symbol type

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
jsdoc: use "prototype X extends Y, contains Z"

Use wording "prototype X extends Y, contains Z", because it is more
accurate then "prototype X inherits Y, Z"

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add doc for Nuvola.KeyValueStorage and children

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>