[VecOps][NFC] Make TVec doc doxygen doc
[VecOps][NFC] Make TVec doc doxygen doc

the documentation was just in a regular C++ multiline block and not
in one recognised by Doxygen.

root-mirror/root,karies/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,karies/root