Since "columns of alignment" are terminated whenever indentation
changes from one line to the next, alignment with spaces will work
independent of the actually chosen tab width. Don't mention tab width
anymore.

Follow-up on https://golang.org/cl/38374/.

For #19618.

Change-Id: I58e47dfde57834f56a98d9119670757a12fb9c41
Reviewed-on: https://go-review.googlesource.com/38379Reviewed-by: Rob Pike <r@golang.org>