blockjob: Improve BlockJobInfo.offset/len documentation

blockjob: Improve BlockJobInfo.offset/len documentation

Clarify that len is just an estimation of the end value of offset, and
that offset increases monotonically while len can change arbitrarily.

While touching the documentation of offset, move it directly after len
to match the order of the declaration below.

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: John Snow <[email protected]>