Improve fs::PathToString documentation

doc/developer-notes.md

@@ -1254,6 +1254,12 @@ A few guidelines for introducing and reviewing new RPC interfaces:
 
   - *Rationale*: User-facing consistency.
 
+- Use `fs::path::u8string()` and `fs::u8path()` functions when converting path
+  to JSON strings, not `fs::PathToString` and `fs::PathFromString`
+
+  - *Rationale*: JSON strings are Unicode strings, not byte strings, and
+    RFC8259 requires JSON to be encoded as UTF-8.
+
 Internal interface guidelines
 -----------------------------