refactor: revisit FullBilloirVertexFitter (#2196)

In this PR we improve the `FullBilloirVertexFitter`.  We resolve issue #2246 and add time to the unit test. In addition, variable names are changed to match the reference, comments improved, and bits of code rewritten.

Core/include/Acts/Vertexing/FullBilloirVertexFitter.hpp

@@ -21,13 +21,21 @@ namespace Acts {
 ///
 /// @brief Vertex fitter class implementing the Billoir vertex fitter
 ///
-/// This class implements the Billoir vertex fitter:
+/// This class implements the Billoir vertex fitter from Ref. (1). It is also
+/// useful to have a look at Ref. (2). Note that the Billoir vertex fitter
+/// outputs one 4D vertex position and nTrack momenta at this very point.
 ///
-/// Fast vertex fitting with a local parametrization of tracks
+/// Ref. (1):
+/// Fast vertex fitting with a local parametrization of tracks.
 /// Author(s) Billoir, P ; Qian, S
 /// In: Nucl. Instrum. Methods Phys. Res., A 311 (1992) 139-150
 /// DOI 10.1016/0168-9002(92)90859-3
 ///
+/// Ref. (2):
+/// Pattern Recognition, Tracking and Vertex Reconstruction in Particle
+/// Detectors.
+/// Author(s) Fruehwirth, R ; Strandli, A
+///
 /// @tparam input_track_t Track object type
 /// @tparam linearizer_t Track linearizer type
 template <typename input_track_t, typename linearizer_t>