add place_varga in API doc

[pierre-haessig]

Hello,

This is a tiny PR proposal to add place_varga to the Function reference/Control system synthesis documentation page.

I guess this was forgotten in #176 when the original place was renamed place_varga. Unless it is intentional?

Also, this may fix the non working of link to place_varga in place doc (in See also section: acker link is fine, but not place_varga ).

[slivingston]

Thanks! Yes, this fixes the link to place_varga. While reviewing this, I noticed the place_varga docstring did not have correct "See Also" numpydoc syntax, so I changed it. (This affects link generation.)

[coveralls]

coverage: 94.503%. remained the same
when pulling c63712e on pierre-haessig:doc-varga
into 2d1f514 on python-control:main.

[pierre-haessig]

Thanks! I had some further ideas that I started in #1006.