encoding/csv: use proper doc comment for Deprecated notes

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>