Fix docstring handling in decorators
Previously, docstrings for solids being created using the decorator syntax were being
overwritten by the docstring for SolidDefinition, making it difficult to document library solids,
solids created by factory functions, etc.
Test Plan: Unit
Reviewers: alangenfeld, prha, schrockn
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D2001