While we're at it, also document Yn(0, 0) = -Inf for completeness.

Fixes #18823.

Change-Id: Ib6db68f76d29cc2373c12ebdf3fab129cac8c167
Reviewed-on: https://go-review.googlesource.com/35970Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>