docs: make kptr_restrict and hash_pointers reference each other

vsprintf.c uses a mix of the `kernel.kptr_restrict` sysctl and the
`hash_pointers` boot param to control pointer hashing. But that wasn't
possible to tell without looking at the source code.

They have a different focus and purpose. To avoid wasting the time of
users trying to use one instead of the other, simply have them reference
each other in the Documentation.

Signed-off-by: Marc Herbert <marc.herbert@linux.intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260107-doc-hash-ptr-v2-1-cb4c161218d7@linux.intel.com>

Author: marc-hb

Documentation/admin-guide/kernel-parameters.txt

@@ -1969,6 +1969,9 @@ Kernel parameters
 				 param "no_hash_pointers" is an alias for
 				 this mode.
 
+			For controlling hashing dynamically at runtime,
+			use the "kernel.kptr_restrict" sysctl instead.
+
 	hashdist=	[KNL,NUMA] Large hashes allocated during boot
 			are distributed across NUMA nodes.  Defaults on
 			for 64-bit NUMA, off otherwise.

Documentation/admin-guide/sysctl/kernel.rst

@@ -591,6 +591,9 @@ if leaking kernel pointer values to unprivileged users is a concern.
 When ``kptr_restrict`` is set to 2, kernel pointers printed using
 %pK will be replaced with 0s regardless of privileges.
 
+For disabling these security restrictions early at boot time (and once
+for all), use the ``hash_pointers`` boot parameter instead.
+
 softlockup_sys_info & hardlockup_sys_info
 =========================================
 A comma separated list of extra system information to be dumped when