`SubmitShares.Success` has unclear description of `new_shares_sum` field

it's not clear what the `new_shares_sum` field of `SubmitShares.Success` really means

after reaching out to @jakubtrnka , he described the following:

> `new_submits_accepted_count` is number of submits.
> `new_shares_sum` is "integral" over difficulty of work to be confirmed by this success msg