message: Document strenghtening
diff: * - RandAddSeedSleep() seeds everything that fast seeding includes, but additionally:
  * - A high-precision timestamp before and after sleeping 1ms.
  * - (On Windows) Once every 10 minutes, performance monitoring data from the OS.
  + - - Once every minute, strengthen the entropy for 10 ms using repeated SHA512.
  * These just exploit the fact the system is idle to improve the quality of the RNG
  * slightly.
  *
  * sources used in the 'slow' seeder are included, but also:
  * - (On Windows) Performance monitoring data from the OS.
  * - (On Windows) Through OpenSSL, the screen contents.
  + * - Strengthen the entropy for 100 ms using repeated SHA512.
  *
  * When mixing in new entropy, H = SHA512(entropy || old_rng_state) is computed, and
  * (up to) the first 32 bytes of H are produced as output, while the last 32 bytes
  