cmd/vet: refer info about -printfuncs to the -printf flag

And vice versa.

The flags are tightly coupled so make the connection clear.

Change-Id: I505f76be631ffa6e489a441c2f3c717aa09ec802
Reviewed-on: https://go-review.googlesource.com/11324Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/vet/doc.go

@@ -50,6 +50,7 @@ with these names, disregarding case:
 	Fatal Fatalf
 	Log Logf
 	Panic Panicf Panicln
+The -printfuncs flag can be used to redefine this list.
 If the function name ends with an 'f', the function is assumed to take
 a format descriptor string in the manner of fmt.Printf. If not, vet
 complains about arguments that look like format descriptor strings.
@@ -174,14 +175,14 @@ These flags configure the behavior of vet:
 	-v
 		Verbose mode
 	-printfuncs
-		A comma-separated list of print-like functions to supplement
-		the standard list.  Each entry is in the form Name:N where N
-		is the zero-based argument position of the first argument
-		involved in the print: either the format or the first print
-		argument for non-formatted prints.  For example,
-		if you have Warn and Warnf functions that take an
-		io.Writer as their first argument, like Fprintf,
+		A comma-separated list of print-like functions to supplement the
+		standard list.  Each entry is in the form Name:N where N is the
+		zero-based argument position of the first argument involved in the
+		print: either the format or the first print argument for non-formatted
+		prints.  For example, if you have Warn and Warnf functions that
+		take an io.Writer as their first argument, like Fprintf,
 			-printfuncs=Warn:1,Warnf:1
+		For more information, see the discussion of the -printf flag.
 	-shadowstrict
 		Whether to be strict about shadowing; can be noisy.
 	-test