chore: addresses review comments

Author: Jakob

include/json/value.h

@@ -585,20 +585,20 @@ class JSON_API Value {
   iterator begin();
   iterator end();
 
-  /// @brief Returns a reference to the first element in the container.
-  /// Calling front on an empty container is undefined behavior.
-  Value const& front() const;
+  /// \brief Returns a reference to the first element in the `Value`.
+  /// Requires that this value holds an array or json object, with at least one element.
+  const Value& front() const;
 
-  /// @brief Returns a reference to the last element in the container.
-  /// Calling back on an empty container is undefined behavior.
-  Value const& back() const;
+  /// \brief Returns a reference to the last element in the `Value`.
+  /// Requires that value holds an array or json object, with at least one element.
+  const Value& back() const;
 
-  /// @brief Returns a reference to the first element in the container.
-  /// Calling front on an empty container is undefined behavior.
+  /// \brief Returns a reference to the first element in the `Value`.
+  /// Requires that this value holds an array or json object, with at least one element.
   Value& front();
 
-  /// @brief Returns a reference to the last element in the container.
-  /// Calling back on an empty container is undefined behavior.
+  /// \brief Returns a reference to the last element in the `Value`.
+  /// Requires that this value holds an array or json object, with at least one element.
   Value& back();
 
   // Accessors for the [start, limit) range of bytes within the JSON text from

src/lib_json/json_value.cpp

@@ -1495,11 +1495,11 @@ Value::iterator Value::end() {
   return iterator();
 }
 
-Value const& Value::front() const { return *begin(); }
+const Value& Value::front() const { return *begin(); }
 
 Value& Value::front() { return *begin(); }
 
-Value const& Value::back() const { return *(--end()); }
+const Value& Value::back() const { return *(--end()); }
 
 Value& Value::back() { return *(--end()); }