StdRng docs refer to non-existent stdrng_* Cargo features

[sfackler]

The current algorithm used is the ChaCha block cipher with either 20 or 12 rounds (see the stdrng_* feature flags, documented in the README).
https://docs.rs/rand/0.7.2/rand/rngs/struct.StdRng.html

Those features don't seem to exist, though.

[dhardy]

No, I proposed them but the idea got rejected, partly because additive feature flags don't correlate well with configuration items, and partly because letting end users make security-performance trade-offs may not be the best idea. See #792.

Somehow the notes in the doc got through though!