dts: binding-template.yaml: Document simple types as well

The int, array, string, and string-array property types were not
documented together with the other types, for whatever reason (might've
been too focused on the more complex types and overlooked it). Add
documentation for them.

Also do some minor cleanup on the descriptions, e.g. to make them more
consistent.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>