clarify choice value restrictions for select and select1

[MartijnR]

I think the only restriction is spaces for a select and for this reason strongly recommending to apply the same no-whitespace-restriction to select1 option values (so that a select1 could easily be changed into a select).

[MartijnR]

ODK has a character restriction of 32 chars for an option value. It might make sense to add this to the spec, as we cannot rely on the client to manage this (since the value is defined in the XForm), as we can for type=string, number etc. And apparently it is important in some cases (server-side submission handling?)