fmt: document verbs %b %d %o %x %X for printing pointers

This has been supported since Go 1 and there's even a test for it.
The documentation was missing.

Fixes #21409.

Change-Id: I5813488f6a98c1b4506c239e968d43344b91be12
Reviewed-on: https://go-review.googlesource.com/59412Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/doc.go

@@ -47,6 +47,8 @@
 		%X	base 16, upper-case, two characters per byte
 	Pointer:
 		%p	base 16 notation, with leading 0x
+		The %b, %d, %o, %x and %X verbs also work with pointers,
+		formatting the value exactly as if it were an integer.
 
 	The default format for %v is:
 		bool:                    %t