Following a new discussion of the format of the text signature, I decided to go with this:
    <name-of-function>(...)\n--\n\n

See here for the discussion:
   https://mail.python.org/pipermail/python-dev/2014-February/132271.html

Patch attached implementing this approach.  Can I get a review?  I want this in before tagging for rc1 on Saturday.

Other changes:

* clinic.py now generates the "/" marker in sigantures to denote
  positional-only parameters.

* clinic.py --make is a lot faster; it prescans the file for any
  clinic block signatures, and if it doesn't see any it skips the file.

* When generating the docstring for a function with optional groups,
  the signature generated is not intended to be machine-readable.
  So it omits the "$", the "/", and the "--" markers.  (See
  Modules/_cursesmodule.c for the one and only example.)