Improve KSS doc for mw-ui-checkbox and input

Improve English, end sentences with a full stop or period, show
placeholder styling in input examples.
Comments only.  `cd docs/kss && make kss && make kssopen` to check.

Change-Id: I61a7c120930b4ba76f9d0e69f36e4ef17db95ac4
Merge "Improve KSS doc for mw-ui-checkbox and input"