strconv: clarify "g" and "G" precision in the docs

Fix the wording in "strconv" and "fmt" to make explicit
that the "g" and "G" formats remove trailing zeroes.

Fixes golang#25082

Change-Id: I2e2ad0a98d2ea27a3a8a006a0563b366f7a3b71b
Reviewed-on: https://go-review.googlesource.com/127135
Reviewed-by: Rob Pike <r@golang.org>

src/fmt/doc.go

@@ -97,10 +97,11 @@
 
 	For floating-point values, width sets the minimum width of the field and
 	precision sets the number of places after the decimal, if appropriate,
-	except that for %g/%G precision sets the total number of significant
-	digits. For example, given 12.345 the format %6.3f prints 12.345 while
-	%.3g prints 12.3. The default precision for %e, %f and %#g is 6; for %g it
-	is the smallest number of digits necessary to identify the value uniquely.
+	except that for %g/%G precision sets the maximum number of significant
+	digits (trailing zeros are removed). For example, given 12.345 the format
+	%6.3f prints 12.345 while %.3g prints 12.3. The default precision for %e, %f
+	and %#g is 6; for %g it is the smallest number of digits necessary to identify
+	the value uniquely.
 
 	For complex numbers, the width and precision apply to the two
 	components independently and the result is parenthesized, so %f applied

src/strconv/ftoa.go

@@ -35,10 +35,11 @@ var float64info = floatInfo{52, 11, -1023}
 // 'g' ('e' for large exponents, 'f' otherwise), or
 // 'G' ('E' for large exponents, 'f' otherwise).
 //
-// The precision prec controls the number of digits
-// (excluding the exponent) printed by the 'e', 'E', 'f', 'g', and 'G' formats.
+// The precision prec controls the number of digits (excluding the exponent)
+// printed by the 'e', 'E', 'f', 'g', and 'G' formats.
 // For 'e', 'E', and 'f' it is the number of digits after the decimal point.
-// For 'g' and 'G' it is the total number of digits.
+// For 'g' and 'G' it is the maximum number of significant digits (trailing
+// zeros are removed).
 // The special precision -1 uses the smallest number of digits
 // necessary such that ParseFloat will return f exactly.
 func FormatFloat(f float64, fmt byte, prec, bitSize int) string {