Add `@@RecordInput` so that docs could be exposed

After this change and run with:
```
bazel run tools/docs:generate --
           --src_dir="$(pwd)/docs_src/"
           --output_dir=/tmp/tfdocs/
```
The documentation correctly shown up in:
`/tmp/tfdocs/api_docs/python/tf/contrib/framework/RecordInput.md`

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>