Documentation: Mention why %p prints ptrval

When debugging recent kernels, people will see '(ptrval)' but there
isn't much information as to what that means. Briefly describe why it's
there.
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Acked-by: NTobin C. Harding <me@tobin.cc>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/printk-formats.rst

@@ -60,8 +60,8 @@ Plain Pointers
 Pointers printed without a specifier extension (i.e unadorned %p) are
 hashed to prevent leaking information about the kernel memory layout. This
 has the added benefit of providing a unique identifier. On 64-bit machines
-the first 32 bits are zeroed. If you *really* want the address see %px
-below.
+the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
+gathers enough entropy. If you *really* want the address see %px below.
 
 Symbols/Function Pointers
 -------------------------