fmt: clarify that for %g precision determines number of significant digits

Documentation change only.

Fixes #15178.

Change-Id: I3c7d80ce9e668ac7515f7ebb9da80f3bd8e534d6
Reviewed-on: https://go-review.googlesource.com/22006Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/doc.go

@@ -95,10 +95,10 @@
 
 	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 it sets the total number of digits. For example,
-	given 123.45 the format %6.2f prints 123.45 while %.4g prints 123.5.
-	The default precision for %e and %f 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 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 and %f 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