add missing documentation on Err case

Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>

Author: dingxiangfei2009

library/core/src/init.rs

@@ -73,7 +73,11 @@ pub unsafe trait Init<T: ?Sized + Pointee> {
  /// Writes a valid value of type `T` to `slot` or fails.
  ///
  /// If this call returns [`Ok`], then `slot` is guaranteed to contain a valid
- /// value of type `T`. If `T` is unsized, then `slot` may be combined with
+ /// value of type `T`.
+ /// Otherwise, in case the result is an [`Err`], the implementation must guarantee
+ /// that the `slot` may be repurposed for future reuse.
+ ///
+ /// If `T` is unsized, then `slot` may be combined with
  /// the metadata to obtain a valid pointer to the value.
  ///
  /// Note that `slot` should be thought of as a `*mut T`. A unit type is used