This patch updates the doc about comments whitespace for the
encoding/csv package to reflect that leading whitespace before
the hash will treat the line as not a comment.

Fixes #13775.

Change-Id: Ia468c75b242a487b4b2b4cd3d342bfb8e07720ba
Reviewed-on: https://go-review.googlesource.com/23302Reviewed-by: Ian Lance Taylor <iant@golang.org>