Document that unknown input parameters are ignored in the REST API

Change-Id: I3d10eae617eeb5b5c767c0ab28753ae7a3a59d02

Documentation/rest-api.txt

diff --git a/Documentation/rest-api.txt b/Documentation/rest-api.txt
index 7928512..8e2bff6 100644
--- a/Documentation/rest-api.txt
+++ b/Documentation/rest-api.txt
@@ -78,6 +78,12 @@
 `Accept-Encoding` request header is set to `gzip`. This may
 save on network transfer time for larger responses.
 
+[[input]]
+=== Input Format
+Unknown JSON parameters will simply be ignored by Gerrit without causing
+an exception. This also applies to case-sensitive parameters, such as
+map keys.
+
 [[timestamp]]
 === Timestamp
 Timestamps are given in UTC and have the format