doc: Improve documentation for 2-D plot functions * doc/interpreter/plot.txi: Rewrite documentation around get/set. * libinterp/interpfcn/graphics.cc(Fget, Fset): Add seealso links. Add additional calling forms for get(). * scripts/plot/bar.m, scripts/plot/barh.m, scripts/plot/comet.m, scripts/plot/comet3.m, scripts/plot/contour.m, scripts/plot/contour3.m, scripts/plot/contourc.m, scripts/plot/contourf.m, scripts/plot/errorbar.m, scripts/plot/gca.m, scripts/plot/gcf.m, scripts/plot/gco.m, scripts/plot/ishghandle.m, scripts/plot/loglogerr.m, scripts/plot/pareto.m, scripts/plot/pcolor.m, scripts/plot/pie.m, scripts/plot/pie3.m, scripts/plot/plotmatrix.m, scripts/plot/plotyy.m, scripts/plot/polar.m, scripts/plot/quiver.m, scripts/plot/quiver3.m, scripts/plot/rose.m, scripts/plot/scatter.m, scripts/plot/semilogxerr.m, scripts/plot/semilogyerr.m, scripts/plot/sombrero.m, scripts/plot/stairs.m, scripts/plot/stem.m, scripts/plot/stem3.m: Improve docstrings.

--- a/scripts/plot/plotyy.m	Sat Jun 22 14:36:42 2013 -0700
+++ b/scripts/plot/plotyy.m	Sat Jun 22 17:40:52 2013 -0700
@@ -51,6 +51,7 @@
 ## ylabel (ax(2), "Axis 2");
 ## @end group
 ## @end example
+## @seealso{plot}
 ## @end deftypefn
 
 function [Ax, H1, H2] = plotyy (varargin)