An option should allow as default `:return:` or `:returns:` in reStructuredText format

[dadadel]

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As @patkujawa-wf and @maqifrnswa pointed, the two syntaxes :return: and :returns: are valid for the reStructuredText format, so it would be nice to accept both and choose the default one via an option. Currently (see issue #9), returns is considered as the good one and any occurrence of :return: will be replaced.