doc: Add gco() to documentation. * doc/interpreter/plot.txi: Add gco() to manual. * scripts/plot/gco.m: Add seealso links to docstring. * scripts/plot/gcbf.m: Re-arrange seealso links. Improve docstring. * scripts/plot/gcbo.m: Rewrite second paragraph of docstring. Add seealso link to gco.

--- a/scripts/plot/gco.m
+++ b/scripts/plot/gco.m
@@ -35,7 +35,7 @@
 ## callback can be interrupted by another callback and the current object
 ## can be modified.
 ##
-##@seealso{gcbo, gcf}
+##@seealso{gcbo, gcf, gca}
 ##@end deftypefn
 
 function h = gco ()