Documentation, mm/unaccepted: document accept_memory kernel parameter

The accept_memory kernel parameter was added in commit dcdfdd40
("mm: Add support for unaccepted memory") but not listed in the
kernel-parameters doc. Add it there.
Acked-by: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231214-accept_memory_param-v2-1-f38cd20a0247@suse.cz

Documentation/admin-guide/kernel-parameters.txt

+	accept_memory=  [MM]
+			Format: { eager | lazy }
+			default: lazy
+			By default, unaccepted memory is accepted lazily to
+			avoid prolonged boot times. The lazy option will add
+			some runtime overhead until all memory is eventually
+			accepted. In most cases the overhead is negligible.
+			For some workloads or for debugging purposes
+			accept_memory=eager can be used to accept all memory
+			at once during boot.
+
 	acpi=		[HW,ACPI,X86,ARM64,RISCV64]
 			Advanced Configuration and Power Interface
 			Format: { force | on | off | strict | noirq | rsdt |