Splat op doc - fix misformat / update tablegen op desc. comment

- bring op description comment in sync with the doc
- fix misformat in doc

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Closes tensorflow/mlir#317

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/317 from bondhugula:quickfix 7fcd945b318c973b2488b702874c87526855c8ef
PiperOrigin-RevId: 285574527