Auto merge of rust-lang#10140 - chansuke:chore/add-comment-for-enclosing-block, r=Alexendoo

chore: add simple comment for `get_enclosing_block`

I was reading the code of `clippy_utils/src/lib.rs` and thought that adding comment on `get_closing_block` would be helpful to first time visitor.

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changelog: none
<!-- changelog_checked -->

Author: bors

clippy_utils/src/lib.rs

@@ -1317,6 +1317,7 @@ pub fn get_parent_expr_for_hir<'tcx>(cx: &LateContext<'tcx>, hir_id: hir::HirId)
     }
 }
 
+/// Gets the enclosing block, if any.
 pub fn get_enclosing_block<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId) -> Option<&'tcx Block<'tcx>> {
     let map = &cx.tcx.hir();
     let enclosing_node = map