docs: Specify default type for sessionPersistence.cookieConfig.lifetimeType (#3540)

Signed-off-by: Arko Dasgupta <[email protected]>

Author: arkodg

apis/v1/shared_types.go

@@ -882,6 +882,8 @@ type CookieConfig struct {
 	// absolute lifetime of the cookie tracked by the gateway and
 	// is optional.
 	//
+	// Defaults to "Session".
+	//
 	// Support: Core for "Session" type
 	//
 	// Support: Extended for "Permanent" type