- In Python, allow multiline documentation on parameters and variables
- Introduce `documentation` attribute on `ParameterNode`, `Parameter` and `Variable` classes
- In the Web API, expose this documentation as a `documentation` property for parameters, variables and variables' formulas
- on `/parameter` nodes as `/parameter/benefits`
> = python `ParameterNode.documentation`
> = YAML parameter node (`index.yaml`) `documentation` string attribute
- on `/parameter` leafs as `/parameter/benefits/housing_allowance`
> = python `Parameter.documentation`
> = YAML parameter `documentation` string attribute
- on `/variable` as `/variable/housing_allowance`
> = python `Variable.documentation`
- on every `/variable` leaf formula
> = python `Variable` formula **docstring**