docs: kdump: Update the crashkernel description for arm64

Now arm64 has added support for "crashkernel=X,high" and
"crashkernel=Y,low". Unlike x86, crash low memory is not allocated if
"crashkernel=Y,low" is not specified.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/20220506114402.365-7-thunder.leizhen@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: Zhen Lei

Documentation/admin-guide/kernel-parameters.txt

@@ -808,7 +808,7 @@
 			Documentation/admin-guide/kdump/kdump.rst for an example.
 
 	crashkernel=size[KMG],high
-			[KNL, X86-64] range could be above 4G. Allow kernel
+			[KNL, X86-64, ARM64] range could be above 4G. Allow kernel
 			to allocate physical memory region from top, so could
 			be above 4G if system have more than 4G ram installed.
 			Otherwise memory region will be allocated below 4G, if
@@ -821,14 +821,19 @@
 			that require some amount of low memory, e.g. swiotlb
 			requires at least 64M+32K low memory, also enough extra
 			low memory is needed to make sure DMA buffers for 32-bit
-			devices won't run out. Kernel would try to allocate at
+			devices won't run out. Kernel would try to allocate
 			at least 256M below 4G automatically.
 			This one let user to specify own low range under 4G
 			for second kernel instead.
 			0: to disable low allocation.
 			It will be ignored when crashkernel=X,high is not used
 			or memory reserved is below 4G.
 
+			[KNL, ARM64] range in low memory.
+			This one lets the user specify a low range in the
+			DMA zone for the crash dump kernel.
+			It will be ignored when crashkernel=X,high is not used.
+
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests