fmt, strconv: document that exponent is always two digits

Except for %b where it is only one.

Fixes #70862

Change-Id: Ic423a799b73bfa534f4083f6544bb9cd639fef06
Reviewed-on: https://go-review.googlesource.com/c/go/+/636915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/fmt/doc.go

@@ -50,6 +50,9 @@ Floating-point and complex constituents:
 	%x	hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20
 	%X	upper-case hexadecimal notation, e.g. -0X1.23ABCP+20
 
+	The exponent is always a decimal integer.
+	For formats other than %b the exponent is at least two digits.
+
 String and slice of bytes (treated equivalently with these verbs):
 
 	%s	the uninterpreted bytes of the string or slice

src/strconv/ftoa.go

@@ -44,6 +44,8 @@ var float64info = floatInfo{52, 11, -1023}
 // zeros are removed).
 // The special precision -1 uses the smallest number of digits
 // necessary such that ParseFloat will return f exactly.
+// The exponent is written as a decimal integer;
+// for all formats other than 'b', it will be at least two digits.
 func FormatFloat(f float64, fmt byte, prec, bitSize int) string {
 	return string(genericFtoa(make([]byte, 0, max(prec+4, 24)), f, fmt, prec, bitSize))
 }