Add information about widths to RegNext (#1318) (#1319)

Adds additional Scaldoc to the RegNext object that (1) indicates that
the width is not set and (2) shows an example of how to construct a
RegNext-like construct with a set width.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
(cherry picked from commit b171f20d

)

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>