def _appendToDocstring(thingWithDoc, textToAppend):
    """
    Append the given text to the docstring of C{thingWithDoc}.

    If C{thingWithDoc} has no docstring, then the text just replaces the
    docstring. If it has a single-line docstring then it appends a blank line
    and the message text. If it has a multi-line docstring, then in appends a
    blank line a the message text, and also does the indentation correctly.
    """
    if thingWithDoc.__doc__:
        docstringLines = thingWithDoc.__doc__.splitlines()
    else:
        docstringLines = []
    if len(docstringLines) == 0:
        docstringLines.append(textToAppend)
    elif len(docstringLines) == 1:
        docstringLines.extend(['', textToAppend, ''])
    else:
        spaces = docstringLines.pop()
        docstringLines.extend(['', spaces + textToAppend, spaces])
    thingWithDoc.__doc__ = '\n'.join(docstringLines)
