Try to hide fwd include in doxygen

[couet]

Try to better fix #8051
In the files Vector2-4-3D.h the definitions in the fwd files are now duplicated,
in order typedef like PtEtaPhiMVector appears defined in Vector4D.h in the ref guide
as it is the file users should include.

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-fedora30/cxx14, ROOT-fedora31/noimt, ROOT-ubuntu16/nortcxxmod, mac1014/python3, mac11.0/cxx17, windows10/cxx14
How to customize builds

[phsft-bot]

Build failed on ROOT-fedora31/noimt.
Running on root-fedora-31-2.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.root.dataframe.roottest_root_dataframe_test_glob

[phsft-bot]

Build failed on ROOT-debian10-i386/cxx14.
Running on pcepsft10.dyndns.cern.ch:/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.root.hist.misc.roottest_root_hist_misc_runownership

[lmoneta]

I understand the PR now fixes the documentation by having the definition of the typedef's directly in Vector.h
However, we are repeating the code, and I am afraid this could cause a problem.
I do not see really an advantage of this. It will add a link to Vector4d.h that anyway does not contain the definition of the templated class that is in LorentzVector.h
The documentation of the class is directly accessible from the typedef definition.
I would then leave the current status and close this PR

[Axel-Naumann]

I am afraid this could cause a problem.

Which one?

As long as users see the template, #include (not the forward!), and documentation when entering PtEtaPhiMVector in the Doxygen search I'll be happy :-)

[lmoneta]

The problem could be that one changes the template class and the typedef declaration in Vector.h but forgets to modify Vectorfwd.h. If that tupedef's is not tested there is an inconsistency.

We have this now https://root.cern.ch/doc/master/namespaceROOT_1_1Math.html#a39def91bfd150148b1534d8ae665b145

[Axel-Naumann]

Simply include ...fwd.h in the ...h header and you're guaranteed that they typedefs are consistent.

[lmoneta]

This is exactly what is now in master and this PR is changing this