Document both valid cases for API iid filter

doc/api/merge_requests.md

@@ -10,7 +10,8 @@ The pagination parameters `page` and `per_page` can be used to restrict the list
 GET /projects/:id/merge_requests
 GET /projects/:id/merge_requests?state=opened
 GET /projects/:id/merge_requests?state=all
-GET /projects/:id/merge_requests?iid[]=42
+GET /projects/:id/merge_requests?iid=42
+GET /projects/:id/merge_requests?iid[]=42&iid[]=43
 ```
 
 Parameters:

doc/api/milestones.md

@@ -6,7 +6,8 @@ Returns a list of project milestones.
 
 ```
 GET /projects/:id/milestones
-GET /projects/:id/milestones?iid[]=42
+GET /projects/:id/milestones?iid=42
+GET /projects/:id/milestones?iid[]=42&iid[]=43
 GET /projects/:id/milestones?state=active
 GET /projects/:id/milestones?state=closed
 ```