Show enum description field on webview documentation.

The enum type's description was missing from generated documentation
because the template only enumerated enum's field names. Showing the
description along with its name when it is available.

BUG=481580

Review URL: https://codereview.chromium.org/1203323002

Cr-Commit-Position: refs/heads/master@{#336267}
1 file changed