Documentation improvement

[Franky4242]

The load function documentation is not complete : the parameter key_is_name is not described.
in load_.py should be :

def load(input_data, key_is_name=False):
    """
    py:function:: piexif.load(filename)
    Return exif data as dict. Keys(IFD name), be contained, are "0th", "Exif", "GPS", "Interop", "1st", and "thumbnail". Without "thumbnail", the value is dict(tag name/tag value). "thumbnail" value is JPEG as bytes.
    :param str filename: JPEG or TIFF
    :param bool key_is_name : if True replaces exif codes by their names as keys in the result dict
    :return: Exif data({"0th":dict, "Exif":dict, "GPS":dict, "Interop":dict, "1st":dict, "thumbnail":bytes})
    :rtype: dict
    """