Improve examples in docstrings

Pull request #433 brings to mind some things that could be improved in docstrings more generally. Cleaning these up might be a good starter issue.

1. Obviously, code examples should be correct and actually work. :)

2. Code examples should also be self-contained if possible, and avoid referring to files in sample-data (most people reading the documentation are probably not running the code directly from the git repository.) In most cases, it's better to use examples from published databases on PhysioNet.

3. Code examples should be written in a consistent style. We generally use numpy style (https://numpydoc.readthedocs.io/en/latest/format.html) for docstrings, but we haven't been entirely consistent. In particular, numpy style requires examples to be written in "doctest" style, like this:

    """
    Examples
    --------
    >>> np.add([[1, 2], [3, 4]],
    ...        [[5, 6], [7, 8]])
    array([[ 6,  8],
           [10, 12]])
    """

This format has some nice properties (it can be tested automatically by doctest, and formatted nicely by sphinx.) On the other hand, the "..." is a little annoying when you want to copy and paste an example.

Empieza revisando el pull request #433 y los docstrings existentes que analiza. Identifica los ejemplos que usan sample-data o no siguen el formato de doctest de NumPy indicado; se considera terminado cuando los ejemplos seleccionados funcionan, son autocontenidos siempre que sea posible, usan datos publicados de PhysioNet cuando corresponde y siguen un estilo coherente.