ErasureCode: proofread abstract API documentation

ErasureCode: proofread abstract API documentation

* Andreas-Joachim Peters suggests to reduce copies to the minimum. When
  possible the output arguments will just point to the input
  argument. This must be documented as any side effect on the input
  argument may modify the output argument

* Fix typos

* Fix may/could/must/should to better reflect what's mandatory and
  what's not.

* Reword the explanation of minimum_to_decode_with_cost to not suggest
  an implementation. This will need to be revisited anyway, when the
  semantic of the cost is defined.

* Explain chunk size constraints

Signed-off-by: Loic Dachary <loic@dachary.org>