docs(plugin): make comments self-contained; drop design-doc and dead-code references

Review the slicing-pipeline plugin comments for context a reader of the source
alone cannot follow, and rewrite them to stand on their own:

- drop pointers to uncommitted design/plan material ("§3.6 (Twistify design)",
  "the brief's note", "Fix 4(a)/4(b)")
- fix dangling references to code this branch removed: the retired set_slices()
  and view mutators, the former G-code post-processing capability/trampoline,
  the "Post-processing" capability family, the pre-refactor array helper
- drop "v1"/"in v1" phase labels, keeping the behavior they described
- correct stale cross-references: Twistify.py -> the real sample path;
  test_plugin_host_api.cpp:32-40 -> import_orca_module in python_test_support.hpp;
  "the binding"/"graphs above" -> the named source

Comment/string-only; no code behavior change.

src/slic3r/plugin/PluginBindingUtils.hpp

@@ -39,8 +39,8 @@ pybind11::array make_readonly_rows(pybind11::handle base, const T* data, pybind1
     namespace py = pybind11;
     if (rows == 0 || data == nullptr) {
         py::array_t<T> empty(std::vector<py::ssize_t>{ 0, (py::ssize_t) N });
-        // Keep behavior-preserving: the pre-refactor helper returned read-only
-        // arrays on every path, so mark the fresh empty array read-only too.
+        // Mark the fresh empty array read-only so every return path of this
+        // helper yields a read-only view.
         empty.attr("setflags")(py::arg("write") = false);
         return std::move(empty);
     }