Documentation: Update the default value of crashkernel low

In commit 94fb933 ("x86/crash: Allocate enough low memory when
crashkernel=high") the default value of crashkernel low memory
is changed to 256M. In this patch update it accordingly.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 22a4b68..945bc0d 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -789,8 +789,10 @@
 			is passed, kernel could allocate physical memory region
 			above 4G, that cause second kernel crash on system
 			that require some amount of low memory, e.g. swiotlb
-			requires at least 64M+32K low memory.  Kernel would
-			try to allocate 72M below 4G automatically.
+			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
+			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.