[skip-ci] improve TPad::Divide help

[couet]

improve TPad::Divide help as requested here #20209

[rlalik]

This part of the documentation is also incorrect:

///  - xmargin is the space along x between pads in percent of canvas.
///  - ymargin is the space along y between pads in percent of canvas.

The more correct could look like:

///  - xmargin is the space around each pad along x, distance between pads along x is equal to 2*xmargin.
///  - ymargin is the space around each pad along y, distance between pads along y is equal to 2*ymargin.

[rlalik]

The new doc sounds really good. I was wondering only about perhaps replacing:

C->SetLeftMargin(0.3);
C->SetRightMargin(0.3);
C->SetTopMargin(0.3);
C->SetBottomMargin(0.3);

with

C->SetMargin(0.3, 0.3, 0.3, 0.3);

The former is very verbose, takes up vertical space. I like the way of setting all margins at once.

But beside that I think this can be merged. Unless you would like to wait for decision of my PR, because this will again change the definition (the old one will stay valid).

[couet]

good I'll merge