Add route MTU document

Signed-off-by: Gris Ge <fge@redhat.com>

Author: cathay4t

devel/yaml_api.md

@@ -1978,6 +1978,9 @@ Each route entry could have these parameters:
    route as `state: absnet` and append new route.
  * `vrf-name`: String. Store the route to the route table which specified VRF
    is binding to.
+ * `mtu`: Integer. New in version 2.2.42. MTU used for this route entry,
+   without `lock-mtu` set to `true`, this MTU will still be altered by kernel
+   Path MTU discovery(PMTUD).
  * `lock-mtu`: Boolean. New in version 2.2.59. When set to true, the route MTU
    is locked, preventing Path MTU Discovery (PMTUD) from automatically
    adjusting it.