Rust unstable book: basic desc and example for const_indexing.

Author: frewsxcv

src/doc/unstable-book/src/const-indexing.md

@@ -6,5 +6,14 @@ The tracking issue for this feature is: [#29947]
 
 ------------------------
 
+The `const_indexing` feature allows the constant evaluation of index operations
+on constant arrays and repeat expressions.
 
+## Examples
 
+```rust
+#![feature(const_indexing)]
+
+const ARR: [usize; 5] = [1, 2, 3, 4, 5];
+const ARR2: [usize; ARR[1]] = [42, 99];
+```