math: fix typos in Bessel function docs

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

Fixes golang#18823.

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

src/math/jn.go

@@ -226,10 +226,10 @@ func Jn(n int, x float64) float64 {
 //
 // Special cases are:
 //	Yn(n, +Inf) = 0
-//	Yn(n > 0, 0) = -Inf
+//	Yn(n ≥ 0, 0) = -Inf
 //	Yn(n < 0, 0) = +Inf if n is odd, -Inf if n is even
-//	Y1(n, x < 0) = NaN
-//	Y1(n, NaN) = NaN
+//	Yn(n, x < 0) = NaN
+//	Yn(n, NaN) = NaN
 func Yn(n int, x float64) float64 {
 	const Two302 = 1 << 302 // 2**302 0x52D0000000000000
 	// special cases