Improve/correct scrypt password hasher documentation

[Dmitry Chestnykh]

The current documentation for scrypt password hasher lacks some constraints and definitions:

1. work_factor: described incompletely, missing the requirement that this value must be a power of 2
2. block_size: has no description
3. parallelism: needs to be defined, perhaps, as "independent computations" rather than "threads", because the current OpenSSL implementation is not multithreaded.

If OpenSSL becomes multithreaded, the "Estimating memory usage" note also needs to be adjusted to account for parallelism.

I think it would also be helpful to mention that these parameters are N, r, p, as these are standard terminology for those familiar with scrypt.

I'll send a pull request shortly.