Explicitly state policy about doc(hidden) things
//! + The crate is continuously developing, and breaking changes are expected
//! during evolution from version to version. We adopt the newest stable
//! rust features if we need them.
+//! + Note that functions/methods/traits/etc. hidden from the docs are not
+//! considered part of the public API, so changes to them are not
+//! considered breaking changes.
//! - Performance:
//! + Prefer higher order methods and arithmetic operations on arrays first,
//! then iteration, and as a last priority using indexed algorithms.
