More clearly distinguish between tab-terminated cells
which are part of an (aligned) column, and non-tab terminated
cells which are not part of a column. Added additional examples.

For #14412.

Change-Id: If72607385752e221eaa2518238b11f48fbcb8a90
Reviewed-on: https://go-review.googlesource.com/19855Reviewed-by: Alan Donovan <adonovan@google.com>