kernel-parameters.txt: document memmap option better

Provide example for memmap exclude option (it is slightly strange and
non-trivial) and provide nice small HOWTO for people with bad memory.
Signed-off-by: NJan-Simon Moeller <dl9pf@gmx.de>
Signed-off-by: NPavel Machek <pavel@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -1130,6 +1130,10 @@ and is between 256 and 4096 characters. It is defined in the file
 	memmap=nn[KMG]$ss[KMG]
 			[KNL,ACPI] Mark specific memory as reserved.
 			Region of memory to be used, from ss to ss+nn.
+			Example: Exclude memory from 0x18690000-0x1869ffff
+			         memmap=64K$0x18690000
+			         or
+			         memmap=0x10000$0x18690000
 
 	meye.*=		[HW] Set MotionEye Camera parameters
 			See Documentation/video4linux/meye.txt.