Rust unstable book: basic desc and example for non_ascii_idents.

Author: frewsxcv

src/doc/unstable-book/src/non-ascii-idents.md

@@ -6,5 +6,13 @@ The tracking issue for this feature is: [#28979]
 
 ------------------------
 
+The `non_ascii_idents` feature adds support for non-ASCII identifiers.
 
+## Examples
 
+```rust
+#![feature(non_ascii_idents)]
+
+const ε: f64 = 0.00001f64;
+const Π: f64 = 3.14f64;
+```