docs(AIP-4233): includes legacy page_size alternatives (#1496)

Author: chalmerlowe

aip/client-libraries/4233.md

@@ -19,6 +19,9 @@ Pagination can be inferred for an RPC when _all_ of the following conditions
 are met:
 
 - The request message contains an `int32 page_size` field.
+  - For APIs that predate [AIP-158][], a field named `max_results` and/or
+    typed as `google.protobuf.UInt32Value` or `google.protobuf.Int32Value`
+    are permissible alternatives.
 - The request message contains a `string page_token` field.
 - The response message contains a `string next_page_token` field.
 - The response message contains one non-primitive `repeated` field.