strconv: document handling of NaN and ±Inf

In addition to the example that was added in 203b80ab, mention these
special cases in the doc comment. This change also adjusts the example
to include "+Inf", as it was not otherwise mentioned that the plus
symbol may be present.

Fix #30990

Change-Id: I97d66f4aff6a17a6ccc0ee2e7f32e39ae91ae454
Reviewed-on: https://go-review.googlesource.com/c/go/+/179738Reviewed-by: Alex Miasoedov <msoedov@gmail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/strconv/atof.go

@@ -654,6 +654,9 @@ func atof64(s string) (f float64, err error) {
 // If s is syntactically well-formed but is more than 1/2 ULP
 // away from the largest floating point number of the given size,
 // ParseFloat returns f = ±Inf, err.Err = ErrRange.
+//
+// ParseFloat recognizes the strings "NaN", "+Inf", and "-Inf" as their
+// respective special floating point values. It ignores case when matching.
 func ParseFloat(s string, bitSize int) (float64, error) {
 	if !underscoreOK(s) {
 		return 0, syntaxError(fnParseFloat, s)

src/strconv/example_test.go

@@ -232,7 +232,7 @@ func ExampleParseFloat() {
 	if s, err := strconv.ParseFloat("inf", 32); err == nil {
 		fmt.Printf("%T, %v\n", s, s)
 	}
-	if s, err := strconv.ParseFloat("Inf", 32); err == nil {
+	if s, err := strconv.ParseFloat("+Inf", 32); err == nil {
 		fmt.Printf("%T, %v\n", s, s)
 	}
 	if s, err := strconv.ParseFloat("-Inf", 32); err == nil {