fmt: document some undocumented details

Better explanation of width for floating-point values.
Explain that scanning does not handle %#q etc.

Fixes #4202.
Fixes #4206.

R=golang-dev, adg, rsc, iant
CC=golang-dev
https://golang.org/cl/6620074

diff --git a/src/pkg/fmt/doc.go b/src/pkg/fmt/doc.go
index a9b9c9d0c27ac2128bafedba1fb64f54a5f4d1ee..8a2289edee49fed6bb4bbef5d5ad1167febb60cd 100644 (file)
--- a/src/pkg/fmt/doc.go
+++ b/src/pkg/fmt/doc.go
@@ -56,9 +56,12 @@
        character '*', causing their values to be obtained from the next
        operand, which must be of type int.
 
-       For numeric values, width sets the width of the field and precision
-       sets the number of places after the decimal, if appropriate.  For
-       example, the format %6.2f prints 123.45.
+       For numeric 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.
 
        For strings, width is the minimum number of characters to output,
        padding with spaces if necessary, and precision is the maximum
@@ -152,6 +155,7 @@
                %T is not implemented
                %e %E %f %F %g %G are all equivalent and scan any floating point or complex value
                %s and %v on strings scan a space-delimited token
+               Flags # and + are not implemented.
 
        The familiar base-setting prefixes 0 (octal) and 0x
        (hexadecimal) are accepted when scanning integers without a