[gpad] clarify in TCanvas docu what decoration is included (#22278)

Co-authored-by: Sergey Linev <S.Linev@gsi.de>

Author: ferdymercury

graf2d/gpad/src/TCanvas.cxx

@@ -112,9 +112,10 @@ of the canvas. It gives a short explanation about the canvas' menus.
 A canvas may be automatically divided into pads via `TPad::Divide`.
 
 At creation time, no matter if in interactive or batch mode, the constructor
-defines the size of the canvas window (including the size of the window
-manager's decoration). To define precisely the graphics area size of a canvas in
-the interactive mode, the following four lines of code should be used:
+defines the size of the canvas window (including the size of the ROOT menu bar but
+excluding the size of the OS window manager's decoration). To define precisely the
+graphics area size of a canvas in batch/interactive mode, the following code should
+be used:
 ~~~ {.cpp}
    {
       Double_t w = 600;
@@ -123,9 +124,18 @@ the interactive mode, the following four lines of code should be used:
       c->SetWindowSize(w + (w - c->GetWw()), h + (h - c->GetWh()));
    }
 ~~~
-and in the batch mode simply do:
+
+To ensure similar painting size for the canvas created with default size for both interactive and batch mode:
+~~~ {.cpp}
+   {
+      auto c = new TCanvas("c", "c");
+      c->SetWindowSize(c->GetWindowWidth() + (c->GetWindowWidth() - c->GetWw()), c->GetWindowHeight() + (c->GetWindowHeight() - c->GetWh())); 
+   }
+~~~
+
+If you are in batch mode, you can also specify the fixed painting area as follows:
 ~~~ {.cpp}
-      c->SetCanvasSize(w,h);
+      c->SetCanvasSize(w, h);
 ~~~
 
 If the canvas size exceeds the window size, scroll bars will be added to the canvas
@@ -1940,8 +1950,8 @@ void TCanvas::SetBatch(Bool_t batch)
 /// are greater than the current canvas window a scroll bar is automatically
 /// generated. Use this function to zoom in a canvas and navigate via
 /// the scroll bars. The Width and Height in this method are different from those
-/// given in the TCanvas constructors where these two dimension include the size
-/// of the window decoration whereas they do not in this method.
+/// given in the TCanvas constructors where these two dimensions include the size
+/// of the ROOT canvas menubar decoration whereas they do not in this method.
 /// When both ww==0 and wh==0, auto resize mode will be enabled again and
 /// canvas drawing area will automatically fit available window size