Clarify docs about Accumulator::update and Accumulator::update_batch

[alamb]

Rationale

As discussed with @realno on #1525 at #1525 (comment)
the distinction between Accumulator::updateandAccumulator::update_batch` was not 100% clear

Changes

Try to clarify the distinction using doc comments

[realno]

Perhaps we can also add the same notes on merge_batch, I think it applies there too.

[alamb]

Perhaps we can also add the same notes on merge_batch, I think it applies there too.

Added in e37679f