docs/kernel-parameters: update description of mem=

The existing description of mem= does not cover all the cases and
differences between how architectures treat it.

Extend the description to match the code.
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20220310082736.1346366-1-rppt@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -2827,6 +2827,9 @@
 
 			For details see: Documentation/admin-guide/hw-vuln/mds.rst
 
+	mem=nn[KMG]	[HEXAGON] Set the memory size.
+			Must be specified, otherwise memory size will be 0.
+
 	mem=nn[KMG]	[KNL,BOOT] Force usage of a specific amount of memory
 			Amount of memory to be used in cases as follows:
 
@@ -2834,6 +2837,13 @@
 			2 when the kernel is not able to see the whole system memory;
 			3 memory that lies after 'mem=' boundary is excluded from
 			 the hypervisor, then assigned to KVM guests.
+			4 to limit the memory available for kdump kernel.
+
+			[ARC,MICROBLAZE] - the limit applies only to low memory,
+			high memory is not affected.
+
+			[ARM64] - only limits memory covered by the linear
+			mapping. The NOMAP regions are not affected.
 
 			[X86] Work as limiting max address. Use together
 			with memmap= to avoid physical address space collisions.
@@ -2844,6 +2854,14 @@
 			in above case 3, memory may need be hot added after boot
 			if system memory of hypervisor is not sufficient.
 
+	mem=nn[KMG]@ss[KMG]
+			[ARM,MIPS] - override the memory layout reported by
+			firmware.
+			Define a memory region of size nn[KMG] starting at
+			ss[KMG].
+			Multiple different regions can be specified with
+			multiple mem= parameters on the command line.
+
 	mem=nopentium	[BUGS=X86-32] Disable usage of 4MB pages for kernel
 			memory.