[CosmosDB] Add new APIversion stable/2020-09-01 (Azure#10915)

* Adds base for updating Microsoft.DocumentDB from version stable/2020-04-01 to version 2020-09-01

* Updates readme

* Updates API version in new specs and examples

* Backup policy changes into new stable version

* Adding default error response for post requests

* Removing analyticalStorageTTL from examples

* Making options optional and setting type for timestamp

specification/cosmos-db/resource-manager/Microsoft.DocumentDB/preview/2020-06-01-preview/cosmos-db.json

@@ -7686,7 +7686,7 @@
         },
         "restoreSource": {
           "type": "string",
-          "description": "Path of the source account from which the restore has to be initiated"
+          "description": "The id of the restorable database account from which the restore has to be initiated. For example: /subscriptions/{subscriptionId}/providers/Microsoft.DocumentDB/locations/{location}/restorableDatabaseAccounts/{restorableDatabaseAccountName}"
         },
         "restoreTimestampInUtc": {
           "type": "string",