doc/go1.17: document strconv changes for Go 1.17

For #44513.
Fixes #46021.

Change-Id: I40a4645fedfae24f67e249743c6a143e71b9f507
Reviewed-on: https://go-review.googlesource.com/c/go/+/326150


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>