Documentation/kernel-parameters.txt: fix memmap= language

Clean up descriptions of memmap= boot options.

Add periods (full stops), drop commas, change "used" to "reserved" or
"marked".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andiry Xu <andiry.xu@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>