Amend value stability / portability rules

[dhardy]

https://rust-random.github.io/book/portability.html

Mentioned: #408, #786

Change: we should permit value-breaking changes in minor releases (e.g. 1.1.0). Such releases are not opted into by default and it seems likely we will want to make such changes eventually without also needing API-breaking changes.

Reference: we link to portability rules in Distribution doc. We should also do so in Rng doc or in rand's crate (top-level) doc.