* update xRooFit to latest version

* declare ROOT::Experimental::XRooFit namespace for ROOT's copy of xRooFit

* fix namespace alias

* address PR comments

* [RF] Improvements to `xroofit` documentation structure

  * Add a new `xroofit` group to the RooFit documentation

  * Add semicolons behind the `NAMESPACE` macros. They have no effect on
    the C++ compilier, but macros without trailing semicolons confuse
    doxygen

  * Add the `xRooNode` and `xRooNLLVar` classes to the `xroofit`
    documentation group. Other xroofit classes might also go there, but
    I don't know which ones should really be public. So I only added
    these two to give and example.

  * Move RooBrowser documentation to xRooBrowser. Now that xRooFit is
    considered part as the experimental user interface, this is fine to
    do. Like this, doxygen can associate the actual xRooBrowser
    implementation with the docs.

  * Rename `RooFit::Detail::XRooFit` namespace to
    `ROOT::Experimental::XRooFit` instad of using namespace alias.
    Again, this is done to not confuse doxygen.

To build the documentation quickly, edit the
`documentation/doxygen/makeinput.sh` directory and comment out all
subdirectories except for `roofit`. Call `make` in the doxygen
directory. Type `y` if it should ask you to overwrite some files in the
end. The xRooFit documentation will end up in the
`~/rootdoc/html/group__xroofit.html` directory.

* address some typos