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: Joel Stanley <[e-mail]>
Acked-by: Tobin C. Harding <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/core-api/printk-formats.rst

···
 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
 -------------------------