clarify that TGaxis func is inverted

[ferdymercury]

trafficstars

Checklist:

• [x] tested changes locally
• [x] updated the docs (if necessary)

This PR fixes https://sft.its.cern.ch/jira/browse/ROOT-8498

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Build failed on ROOT-ubuntu2004/python3. Running on root-ubuntu-2004-3.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.roottest.python.distrdf.dask.roottest_python_distrdf_dask_test_all

[ferdymercury]

pinging @couet