DOC: Add safety documentation on Array trait

Author: bluss

src/array.rs

@@ -12,6 +12,15 @@
 ///   (a few in this range are included by default).
 /// - `array-sizes-129-255`: All sizes 129 to 255 are implemented
 ///   (a few in this range are included by default).
+///
+/// ## Safety
+///
+/// If implemented, this trait can only be implmented by fixed-size arrays or
+/// types with exactly the representation of a fixed size array (of the right
+/// element type and capacity).
+///
+/// Normally this trait is an implementation detail of arrayvec and doesn't
+/// need implementing.
 pub unsafe trait Array {
     /// The array’s element type
     type Item;