Firestore: Add 'FieldPath.documentId()'.#8543

tseaver merged 2 commits intogoogleapis:mastergoogleapis/google-cloud-python:masterfrom

HemangChothani:feature/add_documentId_method_FieldpathHemangChothani/google-cloud-python:feature/add_documentId_method_FieldpathCopy head branch name to clipboard