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/gcf.m
+++ b/scripts/plot/gcf.m
@@ -17,7 +17,7 @@
 ## <http://www.gnu.org/licenses/>.
 
 ## -*- texinfo -*-
-## @deftypefn {Function File} {} gcf ()
+## @deftypefn {Function File} {@var{h} =} gcf ()
 ## Return the current figure handle.  If a figure does not exist, create
 ## one and return its handle.  The handle may then be used to examine or
 ## set properties of the figure.  For example,
@@ -34,7 +34,7 @@
 ## plots a sine wave, finds the handle of the current figure, and then
 ## makes that figure invisible.  Setting the visible property of the
 ## figure to @code{"on"} will cause it to be displayed again.
-## @seealso{get, set}
+## @seealso{gca, gco, get, set}
 ## @end deftypefn
 
 ## Author: jwe, Bill Denney