fmt: tweak the explanation of Stringer to be more correct and less specific

The String method is called whenever the printing operation wants a string,
not just for %s and %v.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8638043

src/pkg/fmt/print.go

@@ -47,7 +47,7 @@ type State interface {
 }
 
 // Formatter is the interface implemented by values with a custom formatter.
-// The implementation of Format may call Sprintf or Fprintf(f) etc.
+// The implementation of Format may call Sprint(f) or Fprint(f) etc.
 // to generate its output.
 type Formatter interface {
 	Format(f State, c rune)
@@ -56,7 +56,8 @@ type Formatter interface {
 // Stringer is implemented by any value that has a String method,
 // which defines the ``native'' format for that value.
 // The String method is used to print values passed as an operand
-// to a %s or %v format or to an unformatted printer such as Print.
+// to any format that accepts a string or to an unformatted printer
+// such as Print.
 type Stringer interface {
 	String() string
 }