add link to 'BorrowKind' in closure.md

Author: JOE1994

src/closure.md

@@ -181,11 +181,12 @@ shared borrow and another one for a mutable borrow. It will also tell us what wa
 
 The callbacks are defined by implementing the [`Delegate`] trait. The
 [`InferBorrowKind`][ibk] type implements `Delegate` and keeps a map that
-records for each upvar which mode of borrow was required. The modes of borrow
-can be `ByValue` (moved) or `ByRef` (borrowed). For `ByRef` borrows, it can be
-`ImmBorrow`, `UniqueImmBorrow`, `MutBorrow` as defined in the
+records for each upvar which mode of capture was required. The modes of capture
+can be `ByValue` (moved) or `ByRef` (borrowed). For `ByRef` borrows, the possible
+[`BorrowKind`][BorrowKind]s are `ImmBorrow`, `UniqueImmBorrow`, `MutBorrow` as defined in the
 [`compiler/rustc_middle/src/ty/mod.rs`][middle_ty].
 
+[BorrowKind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BorrowKind.html
 [middle_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html
 
 `Delegate` defines a few different methods (the different callbacks):