strconv: Document ParseFloat's special cases

Updates #30990

Change-Id: I968fb13251ab3796328089046a3f0fc5c7eb9df9
Reviewed-on: https://go-review.googlesource.com/c/go/+/174204Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/strconv/example_test.go

@@ -222,10 +222,39 @@ func ExampleParseFloat() {
 	if s, err := strconv.ParseFloat(v, 64); err == nil {
 		fmt.Printf("%T, %v\n", s, s)
 	}
+	if s, err := strconv.ParseFloat("NaN", 32); err == nil {
+		fmt.Printf("%T, %v\n", s, s)
+	}
+	// ParseFloat is case insensitive
+	if s, err := strconv.ParseFloat("nan", 32); err == nil {
+		fmt.Printf("%T, %v\n", s, s)
+	}
+	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 {
+		fmt.Printf("%T, %v\n", s, s)
+	}
+	if s, err := strconv.ParseFloat("-Inf", 32); err == nil {
+		fmt.Printf("%T, %v\n", s, s)
+	}
+	if s, err := strconv.ParseFloat("-0", 32); err == nil {
+		fmt.Printf("%T, %v\n", s, s)
+	}
+	if s, err := strconv.ParseFloat("+0", 32); err == nil {
+		fmt.Printf("%T, %v\n", s, s)
+	}
 
 	// Output:
 	// float64, 3.1415927410125732
 	// float64, 3.1415926535
+	// float64, NaN
+	// float64, NaN
+	// float64, +Inf
+	// float64, +Inf
+	// float64, -Inf
+	// float64, -0
+	// float64, 0
 }
 
 func ExampleParseInt() {