doc: Add comments to FIELD_* constants in proxy.h

Author: ViniciusCestarii

include/mp/proxy.h

@@ -304,11 +304,11 @@ struct ProxyServerMethodTraits : public ProxyMethodTraits<MethodParams>
 {
 };
 
-static constexpr int FIELD_IN = 1;
-static constexpr int FIELD_OUT = 2;
-static constexpr int FIELD_OPTIONAL = 4;
-static constexpr int FIELD_REQUESTED = 8;
-static constexpr int FIELD_BOXED = 16;
+static constexpr int FIELD_IN = 1;        //!< Field is read from the Cap'n Proto Params struct (client -> server).
+static constexpr int FIELD_OUT = 2;       //!< Field is read from the Cap'n Proto Results struct (server -> client).
+static constexpr int FIELD_OPTIONAL = 4;  //!< Field has a `has<Field>` sibling in the Cap'n Proto schema; value may be absent and presence must be checked via `has()`.
+static constexpr int FIELD_REQUESTED = 8; //!< Field has a `want<Field>` sibling in the Cap'n Proto Params struct; client opts in to receiving it.
+static constexpr int FIELD_BOXED = 16;    //!< Field is a Cap'n Proto pointer type (struct, list, text, data, interface); value may be absent and presence must be checked via `has()`.
 
 //! Accessor type holding flags that determine how to access a message field.
 template <typename Field, int flags>