Fixed documentation of a few functors

A student asked about the definition of some variables. Since they are often used, I thought just to clarify directly at the functor documentation. Might benefit everybody

Phys/FunctorCore/python/Functors/__init__.py

@@ -157,7 +157,7 @@ PHI_COORDINATE = Functor(
 # same problem as above
 ETA_COORDINATE = Functor(
     'ETA_COORDINATE', "Common::Eta_Coordinate",
-    'Get eta coordinate. Functor to be used in composition.')
+    'Get eta coordinate (pseudorapidity). Functor to be used in composition.')
 RHO_COORDINATE = Functor(
     'RHO_COORDINATE', "Common::Rho_Coordinate",
     'Get rho coordinate. Functor to be used in composition.')
@@ -224,7 +224,7 @@ PHI = setComposedFunctor(
 
 ETA = setComposedFunctor(
     ETA_COORDINATE @ SLOPES, "ETA",
-    """Get the eta coordinate of a slope (see :py:func:`~SLOPES` for definition).
+    """Get the eta (pseudorapidity) coordinate of a slope (see :py:func:`~SLOPES` for definition).
 
     Functor's call expects a track like object""")
 
@@ -597,7 +597,7 @@ def BPVVDRHO(Vertices: DataHandle):
 # why is BPVETA not BPVVDETA?!
 # and why VD instead of FD? all of this seems aribtrary....
 def BPVETA(Vertices: DataHandle):
-    """ Eta coordinate of the distance vector between :py:func:`~ENDVERTEX_POS` and :py:func:`~BPV_POS`
+    """ Eta (pseudorapidity) coordinate of the distance vector between :py:func:`~ENDVERTEX_POS` and :py:func:`~BPV_POS`
 
     Functor's call operator expects a particle like object.
 
@@ -2558,14 +2558,14 @@ _BPVIPCHI2 = Functor(
 
 
 def BPVIPCHI2(Vertices: DataHandle):
-    """Impact parameter with respect to the :py:func:`~BPV`.
+    """Impact parameter Chi2 with respect to the :py:func:`~BPV`. More precisely that means the difference in the chi2 of the PV fit (of the BPV) between including and excluding the object the function is called on.
 
     Functor's call expects a track like object
 
     Args:
       Vertices: DataHandle of the vertices
 
-    Note: Will uses BPV relation if available.
+    Note: Will use BPV relation if available.
     """
     return _BPVIPCHI2.bind(TES(Vertices), FORWARDARGS)