Add documentation for rfl::Commented feature (#600)

Author: liuzicheng1987

README.md

@@ -571,6 +571,7 @@ In addition, it supports the following custom containers:
 - `rfl::Binary`: Used to express numbers in binary format.
 - `rfl::Box`: Similar to `std::unique_ptr`, but (almost) guaranteed to never be null.
 - `rfl::Bytestring`: An alias for `std::vector<std::byte>`. Supported by Avro, BSON, Cap'n Proto, CBOR, flexbuffers, msgpack and UBJSON. 
+- `rfl::Commented`: Allows you to add comments to fields (supported by YAML and XML).
 - `rfl::Generic`: A catch-all type that can represent (almost) anything.
 - `rfl::Hex`: Used to express numbers in hex format.
 - `rfl::Literal`: An explicitly enumerated string.

docs/commented.md

@@ -0,0 +1,81 @@
+# `rfl::Commented`
+
+The `rfl::Commented<T>` wrapper allows you to add comments to fields in your structs. These comments are then serialized in formats that support them, such as YAML and XML.
+
+Note that `rfl::Commented` is currently unsupported by formats that do not have a standard way of representing comments, such as JSON. Also note that comments are **write-only**: they are ignored during deserialization.
+
+## Example (YAML)
+
+In YAML, the comments are added as line comments (`#`):
+
+```cpp
+struct Person {
+  std::string first_name;
+  std::string last_name;
+  rfl::Commented<std::string> town;
+};
+
+const auto homer = Person{.first_name = "Homer",
+                          .last_name = "Simpson",
+                          .town = rfl::Commented<std::string>(
+                              "Springfield", "The town where Homer lives")};
+
+const auto yaml_str = rfl::yaml::write(homer);
+```
+
+This will result in the following YAML:
+
+```yaml
+first_name: Homer
+last_name: Simpson
+town: Springfield  # The town where Homer lives
+```
+
+## Example (XML)
+
+In XML, the comments are added as `<!-- comment -->` blocks after the field:
+
+```cpp
+struct Person {
+  std::string first_name;
+  std::string last_name;
+  rfl::Commented<std::string> town;
+};
+
+const auto homer = Person{.first_name = "Homer",
+                          .last_name = "Simpson",
+                          .town = rfl::Commented<std::string>(
+                              "Springfield", "The town where Homer lives")};
+
+const auto xml_str = rfl::xml::write(homer);
+```
+
+This will result in the following XML:
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<Person>
+    <first_name>Homer</first_name>
+    <last_name>Simpson</last_name>
+    <town>Springfield</town>
+    <!--The town where Homer lives-->
+</Person>
+```
+
+## API convenience
+
+`rfl::Commented` provides several ways to access and modify the underlying value and the comment:
+
+- `.get()`, `.value()`, `operator()()` — access the underlying value (const and non-const overloads).
+- `.comment()` — returns an `std::optional<std::string>` containing the comment.
+- `.add_comment(std::string)` — sets or updates the comment.
+- `.set(...)`, `operator=(...)` — assign the underlying value.
+
+Example:
+
+```cpp
+Person p;
+p.town = "Springfield";
+p.town.add_comment("The town where Homer lives");
+std::string s = p.town.value();
+```

mkdocs.yaml

@@ -123,6 +123,7 @@ nav:
       - rfl::Box and rfl::Ref: rfl_ref.md
       - rfl::Timestamp: timestamps.md
       - rfl::Skip: rfl_skip.md
+      - rfl::Commented: commented.md
       - rfl::Result: result.md
       - Standard containers: standard_containers.md
       - C arrays and inheritance: c_arrays_and_inheritance.md