Clarify ambiguities around results order and silent option

This pull request makes two major changes:

* Clarify that results come back in the same order as their corresponding requests were specified (e.g. requests [A, B, C] will generate responses [A', B', C']), for both sequential and parallel processing.
* Clarify that the `silent` option will result in a null value being inserted into the results array, preserving the array indexes of subsequent responses.

This also includes some inconsequential formatting and wording tweaks.