Documentation for IEC binary and SI decimal prefix handling

Documentation for IEC binary and SI decimal prefix handling

Documentation for the previous patch improving IEC and SI
decimal prefix handling (e.g., KiB, MiB, GiB vs. KB, MB, GB).

Includes updates to int, kb_base, and the parameters using
int to express quantities of data (e.g., bs, ba, bsrange, bssplit).

Includes updates to parameters that accept comma-separated
values (usually ints) for reads, writes, and trims; trims were
often not mentioned.

Signed-off-by: Jens Axboe <axboe@fb.com>