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/PluginManager.hpp

@@ -102,7 +102,7 @@ void execute_capabilities_from_refs(const ConfigOptionStrings& capabilities,
 {
     PluginManager&                plugin_mgr = PluginManager::instance();
 
-    // Log prefix derived from the capability type so each capability family (Post-processing,
+    // Log prefix derived from the capability type so each capability family (Printer connection,
     // Slicing Pipeline, ...) tags its dispatch diagnostics with its own display name.
     const std::string tag = plugin_capability_type_display_name(type);