Update docs of validate_string to reflect bug

[JimPanic]

See: https://tickets.puppetlabs.com/browse/MODULES-457

[hunner]

It still appears to pass undef as strings on the future parser:

extern% puppet apply -e 'validate_string(undef)' --parser=future
Notice: Compiled catalog for extern.corp.puppetlabs.net in environment production in 0.44 seconds
Notice: Finished catalog run in 0.01 seconds

[zaphod42]

Yes, functions written using the old API will still receive "" under the future parser. Functions written with the new API will get a ruby nil for undef.

[zaphod42]

We didn't change the semantics of undef under the old API because we didn't want to break existing functions that might (perhaps inadvertently) be relying on getting "" instead of nil.

[JimPanic]

It would be very much appreciated if this was reflected in the documentation.