Add note on retrying RPCs 

Also add keepalive to list.
diff --git a/doc/statuscodes.md b/doc/statuscodes.md
index f2df9e0..9fbd58b 100644
--- a/doc/statuscodes.md
+++ b/doc/statuscodes.md
@@ -38,6 +38,7 @@
 | Error parsing response proto	| INTERNAL | Client|
 | Error parsing request proto	| INTERNAL | Server|
 | Sent or received message was larger than configured limit | RESOURCE_EXHAUSTED | Both |
+| Keepalive watchdog times out | INTERNAL | Both |
 
 The following status codes are never generated by the library:
 - INVALID_ARGUMENT
@@ -47,3 +48,5 @@
 - ABORTED
 - OUT_OF_RANGE
 - DATA_LOSS
+
+The decision to retry RPCs at the application level depends on the application and the type of error. There is no single guidance that will work for all.