[interop test docs] document --additional_metadata flag (#33180)

The flag was introduced a while back for C++:
https://github.com/grpc/grpc/pull/18156

https://github.com/grpc/grpc-go/pull/6295 and
https://github.com/grpc/grpc-java/pull/10201 for the same flag in Java
and Go (need this for RLS integration tests).

doc/interop-test-descriptions.md

@@ -39,6 +39,18 @@ Clients should accept these arguments:
 * --service_config_json=SERVICE_CONFIG_JSON
     * Disables service config lookups and sets the provided string as the
       default service config.
+* --additional_metadata=ADDITIONAL_METADATA
+    * Additional metadata to send in each request, as a semicolon-separated list
+      of key:value pairs. The first key/value pair is separated by the first colon.
+      The second key/value pair is separated by the next colon *following* the
+      next semi-colon thereafter, and so on. For example:
+      - `abc-key:abc-value;foo-key:foo-value`
+          - Key/value pairs: `abc-key`/`abc-value`, `foo-key`/`foo-value`.
+      - `abc-key:abc:value;foo-key:foo:value`
+          - Key/value pairs: `abc-key`/`abc:value`, `foo-key`/`foo:value`.
+
+      Keys must be ASCII only (no `-bin` headers allowed). Values may contain
+      any character except semi-colons.
 
 Clients must support TLS with ALPN. Clients must not disable certificate
 checking.