Add caption formatting to css.

BUG=none
R=
TEST=none
NOTRY=true
(documentation only change)

Review URL: https://codereview.chromium.org/587753003

Cr-Commit-Position: refs/heads/master@{#297441}

chrome/common/extensions/docs/static/sass/_typography.scss

@@ -73,6 +73,8 @@ p {
   }
   &.caption {
     text-align: left;
+    margin-top: 9px;
+    font-weight: bold;
     .lightbox & {
       color: $white;
     }