Document the ordering behavior of crate cfgs

This was changed in rust-lang/rust#110141.

Author: riking

src/conditional-compilation.md

@@ -254,6 +254,12 @@ It is written as `cfg`, `(`, a configuration predicate, and finally `)`.
 If the predicate is true, the thing is rewritten to not have the `cfg` attribute
 on it. If the predicate is false, the thing is removed from the source code.
 
+When a crate-level `cfg` evaluates to false, the behavior is slightly
+different: any crate attributes preceding the `cfg` are kept, and any crate
+attributes following the `cfg` are removed. This allows `#![no_std]` and
+`#![no_core]` crates to avoid linking `std`/`core` even if a `#![cfg(...)]` has
+removed the entire crate.
+
 Some examples on functions:
 
 ```rust