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/ishghandle.m
+++ b/scripts/plot/ishghandle.m
@@ -18,7 +18,10 @@
 
 ## -*- texinfo -*-
 ## @deftypefn {Function File} {} ishghandle (@var{h})
-## Return true if @var{h} is a graphics handle and false otherwise.
+## Return true if @var{h} is a graphics handle and false otherwise.  This
+## function is equivalent to @code{ishandle} and is provided for compatibility
+## with @sc{matlab}.
+## @seealso{ishandle}
 ## @end deftypefn
 
 function retval = ishghandle (h)