fmt: document %%

R=rsc, gri
CC=golang-dev
https://golang.org/cl/4130044

src/pkg/fmt/doc.go

@@ -16,6 +16,7 @@
 			when printing structs, the plus flag (%+v) adds field names
 		%#v	a Go-syntax representation of the value
 		%T	a Go-syntax representation of the type of the value
+		%%	a literal percent sign; consumes no value
 
 	Boolean:
 		%t	the word true or false