Description
Is your feature request related to a problem? Please describe.
Add docstrings to PDL Block definitions for generated docs so that https://ibm.github.io/prompt-declaration-language/api_reference/ eventually describes every attribute a PDL developer can use in a PDL program.
Additional context
It is frustrating to see, for example, https://ibm.github.io/prompt-declaration-language/api_reference/#src.pdl.pdl_ast.LitellmModelBlock which says "Bases: ModelBlock" but ModelBlock is not defined anywhere.
ModelBlock has attributes
kind: Literal[BlockKind.MODEL] = BlockKind.MODEL
model: ExpressionType
input: Optional["BlockType"] = None
trace: Optional["BlockType"] = None
modelResponse: Optional[str] = None
input isn't documented on https://ibm.github.io/prompt-declaration-language/api_reference . It isn't clear to me if the model defined in ModelBlock and the model defined in LitellmModelBlock are the same, or if one hides the other.