Fix some bogosities in geometric-function documentation: add an entry

Fix some bogosities in geometric-function documentation: add an entry
for circle(polygon), which was missing; remove bogus entry for
point(lseg, lseg), which does not exist, and the documentation seemed to
describe lseg_interpt, which we already document as an operator not a
function.  Also remove entry for box_intersect, which likewise is
preferentially used via the operator #.