Improve docstrings

[znamensk]

At a minimum, all docstrings should include a list of function arguments and returned values. Docstrings should follow the Google style and can include references to be parsed by sphinx (e.g. :py:class:pandas.DataFrame):

    """
    Get entities of a given type and format results.

    Entities can be filtered by name, id, origin, or attribute (using the
    `query_key` / `query_value` arguments).

    Args:
        datatype (str): type of Flexylims entity to fetch, e.g. 'mouse', 'session',
            'recording', or 'dataset'. This is the only mandatory argument.
        query_key (str): attribute to filter by.
        query_value (str): attribute value to select
        project_id (str): text name of the project. Either `project_id` or
            `flexilims_session` must be provided.
        flexilims_session (:py:class:`flexilims.Flexilims`): Flexylims session
            object. This is preferred to providing `project_id` as it avoids
            creating new authentication tokens.
        name (str): filter by name
        origin_id (str): filter by origin / parent
        id (str): filter by hexadecimal id
        format_reply (bool): (default True) whether to format the reply into a
            `Dataframe`. If this is set to false, a list of dictionaries will be
            returned instead.

    Returns:
        :py:class:`pandas.DataFrame`: containing all matching entities

    """

For methods extensively used by end users, it would also be great to include examples:

    Examples:
        Examples should be written in doctest format, and should illustrate how
        to use the function.

        >>> print([i for i in example_generator(4)])
        [0, 1, 2, 3]