Update compiler/rustc_mir_transform/src/patch.rs

Co-authored-by: lcnr <rust@lcnr.de>

compiler/rustc_mir_transform/src/patch.rs

@@ -23,6 +23,8 @@ pub(crate) struct MirPatch<'tcx> {
     terminate_block: Option<(BasicBlock, UnwindTerminateReason)>,
     body_span: Span,
     next_local: usize,
+    /// The number of blocks at the start of the transformation. New blocks
+    /// get appended at the end.
     next_block: usize,
 }