DOC: special: Added docstrings for gdtria, gdtrib and gdtrix.

The terse docstring style of the scipy.special ufunc docstrings
is maintained, but they include examples.
DOC: special: write proper docstring for gdtria, gdtrib, gdtrix.
DOC: special: add a description of the optional fourth argument for gdtria, gdtrib and gdtrix.
DOC: special: add the function signature to the top of the docstrings for gdtria, gdtrib and gdtrix.
Merge pull request #459 from WarrenWeckesser/special-gdtri-docstrings

DOC: special: Added docstrings for gdtria, gdtrib and gdtrix.