Add feature flag to reference docs for RFC 1623.

Author: chriskrycho

src/doc/reference.md

@@ -1356,14 +1356,15 @@ unsafe fn bump_levels_unsafe2() -> u32 {
 Mutable statics have the same restrictions as normal statics, except that the
 type of the value is not required to ascribe to `Sync`.
 
-#### `'static` lifetime elision
+#### `'static` lifetime elision [unstable]
 
 Both constant and static declarations of reference types have *implicit*
 `'static` lifetimes unless an explicit lifetime is specified. As such, the
 constant declarations involving `'static` above may be written without the
 lifetimes. Returning to our previous example:
 
 ```rust
+#[feature(static_in_const)]
 const BIT1: u32 = 1 << 0;
 const BIT2: u32 = 1 << 1;