Documentation: complete crashkernel= parameter documentation

Complete the crashkernel= kernel parameter documentation.

Signed-off-by: Ahmed S. Darwish <[e-mail]>
Acked-by: Simon Horman <[e-mail]>
Acked-by: Vivek Goyal <[e-mail]>
Signed-off-by: Randy Dunlap <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/kernel-parameters.txt

···
 			Format:
 			<first_slot>,<last_slot>,<port>,<enum_bit>[,<debug>]
 
-	crashkernel=nn[KMG]@ss[KMG]
-			[KNL] Reserve a chunk of physical memory to
-			hold a kernel to switch to with kexec on panic.
+	crashkernel=size[KMG][@offset[KMG]]
+			[KNL] Using kexec, Linux can switch to a 'crash kernel'
+			upon panic. This parameter reserves the physical
+			memory region [offset, offset + size] for that kernel
+			image. If '@offset' is omitted, then a suitable offset
+			is selected automatically. Check
+			Documentation/kdump/kdump.txt for further details.
 
 	crashkernel=range1:size1[,range2:size2,...][@offset]
 			[KNL] Same as above, but depends on the memory
 			in the running system. The syntax of range is
 			start-[end] where start and end are both
 			a memory unit (amount[KMG]). See also
-			Documentation/kdump/kdump.txt for a example.
+			Documentation/kdump/kdump.txt for an example.
 
 	cs89x0_dma=	[HW,NET]
 			Format: <dma>