docs issues

[Petr M. <mi...@ph...>]

According to the past discussion, I propose to change the following issues 
in the docs:


(A)
3 dgrid3d
 The `set dgrid3d` command enables, and can set parameters for, non-grid
 to grid data mapping.
=>
 The `set dgrid3d` command enables, and can set parameters for, non-grid
 to grid data mapping. See `splot grid_data` for more details about the
 grid data structure.


(B)
3 contour
 `set contour` enables contour drawing for surfaces. This option is 
 available for `splot` only.
 ...
 If contours are desired from non-grid data, `set dgrid3d` can be used to
 create an appropriate grid.  See `set dgrid3d` for more information.
=>
 `set contour` enables contour drawing for surfaces. This option is 
 available for `splot` only. It requires grid data, see `grid_data` for more 
 details. If contours are desired from non-grid data, `set dgrid3d` can be 
 used to create an appropriate grid.


(C)
  There was some discussion on this list about the differences of
	splot 'bla' vs splot 'bla' using 1:2:3
  but I cannot find it in the docs. Could somebody summarize it there,
  and cross-link it with "help missing"?
  Further, I propose to label "help using" by "help nan" as well.

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PM