Add some description before calling the function to read the comment in py files

[javiergarciaheras]

Add description before presenting the comment in the functions in documentation/code.md

Optical Flow

include here some description about optical flow

.. automodule:: metintos.flow_tuning
:members: add_times_to_steps_sources_targets, add_steps_to_times_sources_targets, filter_dims, evaluate_flow_on_dataset, evaluate_flow_on_dataset_norms, optimize_flow

.. autodata:: metintos.flow_tuning.TimeSelector

.. automodule:: metintos.optiflow
:members:
:special-members: init, call