update Types doc to clarify "sets" are supported

After this paragraph >>

A type defines the set of values and operations on those values. Types are never explicitly declared as part of the syntax except as part of a [builtin statement](https://docs.influxdata.com/flux/v0.x/spec/system-built-ins/). Types are always inferred from the usage of the value. Type inference follows a Hindley-Milner style inference system.

add the following >>

Flux supports the following types (and insert links):

- Union types
- Basic types (null, boolean, numeric, time, duration, string, and bytes)
- Regular expression
- Composite types (types for "sets" of data, including array, record, and dictionary; function and generator)
- Polymorphism (parametric, record, ad hoc; type constraints to implement ad hoc polymorphism)