Nightly build and stable differences

[evelynmitchell]

Add mention of differences between nightly build and stable:

skade (#rust-docs): "I see people constantly falling into the trap that "nightly is just stable with features" (like, LLVM assertions activated in nightly, adding to compile times), well, there's also misconceptions about the level of stability features have (like, some have: "never") written next to them, etc., also that there are two kind of feature flags and codegen features usually hide behind -Z

Migration from rust-lang/rust#40079

[steveklabnik]

@carols10cents this is something I feel like we've missed out on in the current draft; I need to re-read chapter 1 and see though. But knowing this stuff is an important part of doing Rust development.