Improvements and perspective from architecture review

docs/document-functions.md

@@ -50,6 +50,12 @@ function:
 {{mkt:text.upper "draft" | text.replace DRAFT Final}}
 ```
 
+Quoted pipe characters remain literal:
+
+```markdown
+{{mkt:text.replace "a|b" "|" "/"}}
+```
+
 Values of the form `${name}` are resolved from `ProcessingContext.variables`.
 This keeps data binding aligned with workflow expression conventions without
 creating a second workflow engine.
@@ -126,6 +132,24 @@ capabilities before execution. External policy services may provide decisions
 through adapters later, but deterministic function execution has no external
 service dependency.
 
+## Natural Extensions
+
+The deterministic layer deliberately stops before becoming a full publishing
+language. Future extension work is captured in
+`MKTT-WP-0015: Render And Document Function Extensions`.
+
+That workplan should consider:
+
+- typed document values and value-to-Markdown mapping
+- richer multiline and nested function syntax
+- document-local reusable functions
+- render/export adapters, including optional Quarkdown source export
+- render-aware numbering, references, tables, figures, equations, and code
+  blocks
+- static asset and media manifests with checksums
+- local permission gates for filesystem, network, external process, assisted,
+  and render/export functions
+
 ## Design Rules
 
 - Stay close to Markdown and preserve CommonMark documents unless function