Add comment for future_incompatible lint group

compiler/rustc_lint/src/context.rs

@@ -221,6 +221,9 @@ impl LintStore {
                         .lint_ids
                         .push(id);
                 } else {
+                    // Lints belonging to the `future_incompatible` lint group are lints where a
+                    // future version of rustc will cause existing code to stop compiling.
+                    // Lints tied to an edition don't count because they are opt-in.
                     self.lint_groups
                         .entry("future_incompatible")
                         .or_insert(LintGroup {