fmt: document %b for floating point

R=rsc
CC=golang-dev
https://golang.org/cl/4073051

src/pkg/fmt/doc.go

@@ -28,6 +28,8 @@
 		%X	base 16, with upper-case letters for A-F
 		%U	Unicode format: U+1234; same as "U+%x" with 4 digits default
 	Floating-point and complex constituents:
+		%b	decimalless scientific notation with exponent a power
+			of two, in the manner of strconv.Ftoa32, e.g. -123456p-78
 		%e	scientific notation, e.g. -1234.456e+78
 		%E	scientific notation, e.g. -1234.456E+78
 		%f	decimal point but no exponent, e.g. 123.456