Clean up descriptions of memmap= boot options.

Add periods (full stops), drop commas, change "used" to "reserved" or
"marked".
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Cc: Andiry Xu <andiry.xu@gmail.com>
Acked-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>