Add has_docstring param to expressify()

[wch]

Closes #1081.

This allows the user to tell @expressify that the decorated function has a docstring which shouldn't be displayed. For example:

@expressify(has_docstring=True)
def f():
    "This is a docstring. It should not be displayed."
    "This is a regular string. It should be displayed."

f()

I don't love this implementation, and am open to ideas about how to do it more cleanly.