cmd/gofmt: clarify doc string even more

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>

src/cmd/gofmt/doc.go

@@ -5,9 +5,7 @@
 /*
 Gofmt formats Go programs.
 It uses tabs for indentation and blanks for alignment.
-
-Alignment assumes that an editor is using a fixed-width
-font and that a tab corresponds to 8 spaces.
+Alignment assumes that an editor is using a fixed-width font.
 
 Without an explicit path, it processes the standard input.  Given a file,
 it operates on that file; given a directory, it operates on all .go files in