[Feature] Document functions in Puppet Datatypes

[glennsarti]

Use Case

In Pull Request #199 only Datatype attributes were added. Datatype functions should also be documented

Describe the Solution You Would Like

Puppet-strings should document data type functions like a regular function

Describe Alternatives You've Considered

N/A

Additional Context

The Bolt core libraries use functions e.g.
https://github.com/puppetlabs/bolt/blob/master/bolt-modules/boltlib/lib/puppet/datatypes/target.rb