rgw: document convention used in ReadOp::read and ReadOp::iterate

rgw: document convention used in ReadOp::read and ReadOp::iterate

Both of these functions defined at the SAL layer take an offset and an
end offset. It's important to note that the end offset is inclusive
since a common, modern convention is for the end offset to be
exclusive.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>