Add comment to cleanup_kinds

based on the original commit message 1ae7ae0c1c

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -261,6 +261,9 @@ pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
     }
 }
 
+/// MSVC requires unwinding code to be split to a tree of *funclets*, where each funclet can only
+/// branch to itself or to its parent. Luckily, the code we generates matches this pattern.
+/// Recover that structure in an analyze pass.
 pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
     fn discover_masters<'tcx>(
         result: &mut IndexVec<mir::BasicBlock, CleanupKind>,