[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.