Add missing documentation

Signed-off-by: Michel Hidalgo <[email protected]>

io/include/gz/common/DataFrame.hh

@@ -41,19 +41,28 @@ namespace gz
  template <typename K, typename V>
  class DataFrame
  {
- /// \brief Check if column key is present.
+ /// \brief Check if column is present.
+ /// \param[in] _key Key to column to look up.
+ /// \return whether the given column is present
+ /// in the data frame.
  public: bool Has(const K &_key) const
  {
  return this->storage.count(_key) > 0;
  }
 
- /// \brief Fetch mutable reference to column
+ /// \brief Fetch mutable reference to column.
+ /// \param[in] _key Key to column to look up.
+ /// \return Mutable reference to column in the
+ /// data frame.
  public: V &operator[](const K &_key)
  {
  return this->storage[_key];
  }
 
  /// \brief Fetch immutable reference to column
+ /// \param[in] _key Key to column to look up.
+ /// \return Immutable reference to column in the
+ /// data frame.
  public: const V &operator[](const K &_key) const
  {
  return this->storage.at(_key);