Add missing doc line

Author: mikoarce

API_DOCS.md

@@ -155,6 +155,7 @@ resources:
             isBatchKeyASet: ?boolean          (can only use if isBatchResource=true)
             propertyBatchKey: ?string         (can only use if isBatchResource=true)
             responseKey: ?string              (non-optional when propertyBatchKey is used)
+            maxBatchSize: ?integer            (can only use if isBatchResource=true)
 
 typings:
     language: flow