Update THStack documentation

[couet]

As said in this old forum post https://root-forum.cern.ch/t/thstack-th2-with-negative-values/16161/5, stacking a histogram with negative bin content does not make sense. This issue is not relevant #15740. But it is goo to make it clear in the documentation and to prevent adding such histograms in a stack. That's what this PR is doing.

[linev]

Therefore, histograms with a negative minimum will be rejected.

How it is done? Currently I see no protection for negative bins content.

[github-actions]

Test Results

    11 files      11 suites   2d 12h 14m 46s ⏱️
 2 645 tests  2 645 ✅ 0 💤 0 ❌
27 455 runs  27 455 ✅ 0 💤 0 ❌

Results for commit a76d35a.

♻️ This comment has been updated with latest results.

[couet]

Therefore, histograms with a negative minimum will be rejected.

How it is done? Currently I see no protection for negative bins content.

In Add:

if (h1->GetMinimum() < 0.) {
       Error("Add","Histograms with a negative minimum cannot be part of a THStack");
       return;
}

[couet]

Any way this PR make several test fail like "tutorial-legacy-mlp-mlpRegression" let see what @moneta think about it.

[couet]

It seems OK now

[linev]

One should change Warning method name.