encoding/csv: use proper doc comment for Deprecated notes
    
    End-of-line comments are not doc comments,
    so Deprecated notes in them are not recognized
    as deprecation notices. Rewrite the comments.
    
    Change-Id: I275fa9aec403132fda45853e52daa22bc06fcd36
    Reviewed-on: https://go-review.googlesource.com/c/go/+/453617
    Reviewed-by: Bryan Mills <bcmills@google.com>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Auto-Submit: Russ Cox <rsc@golang.org>
    Run-TryBot: Russ Cox <rsc@golang.org>
    Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
    Reviewed-by: Ian Lance Taylor <iant@google.com>