Update docs now that we removed captions from screenshots (bug 920220)

docs/api/topics/apps.rst

@@ -75,7 +75,6 @@ App
             "premium_type": "premium",
             "previews": [
                 {
-                    "caption": "",
                     "filetype": "image/png",
                     "id": "37",
                     "image_url": "/tmp/uploads/previews/full/0/37.png?modified=1362762723",

docs/api/topics/submission.rst

@@ -260,7 +260,6 @@ Screenshots or videos
             "image_url": "/img/uploads/...",
             "filetype": "image/png",
             "resource_uri": "/api/v1/apps/preview/1/"
-            "caption": "Awesome screenshot"
         }
 
 .. http:delete:: /api/v1/apps/preview/(int:preview_id)/