HOWTO: minor fixes and backports from man page

These are minor fixes not worth separating into different commits,
which are partly backports from the previous 3 commits, and partly
its own minor fixes.

 * Use proper argument name (option -> command).
 * Use proper quotation (``).
 * Add missing :option:`` directives.
 * Add missing :file:`` directive (stat.h).
 * Add indentation.
 * Add missing alias (mem_align=).
 * Add missing period.
 * Add $ for fio command line examples
 * Cleanup ordering of some ioengine specific options.
 * Change inconsistent quotations for "1..N" and "N" to always use ``.
 * Avoid mixing of whitespace and tab indentation within the same section/block.
 * Correct fio commands' name (fio-genzipf, fio_generate_plots)
 * Correct typos ("used,where", "See disable_slat" in disable_slat)
 * Change "IO" to "I/O" which has been corrected in the past commits as well,
   except for "IO" in the output of fio itself.
 * Mention verification trigger (using magic sysrq) is Linux specific.
 * Mention "Disk stats" output (using sysfs) is Linux specific.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>