cmd/gofmt: clarify documentation re: tab width

Fixes #19618.

Change-Id: I0ac450ff717ec1f16eb12758c6bf5e98b5de20e8
Reviewed-on: https://go-review.googlesource.com/38374Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/gofmt/doc.go

@@ -4,7 +4,10 @@
 
 /*
 Gofmt formats Go programs.
-It uses tabs (width = 8) for indentation and blanks for alignment.
+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.
 
 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