[hist] Clarify that TGraph/TH1::Fit(name) are not thread safe.

Co-authored-by: ferdymercury <ferdymercury@users.noreply.github.com>

Author: hageboeck

hist/hist/src/TGraph.cxx

@@ -1267,14 +1267,17 @@ TFitResultPtr TGraph::Fit(TF1 *f1, Option_t *option, Option_t *goption, Axis_t r
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-/// Fit this graph with function with name `fname`.
+/// Fit this graph with the global function named `fname`.
 ///
-/// This is a different interface to TGraph fitting using TGraph::Fit(TF1 *f1,Option_t *, Option_t *, Axis_t, Axis_t)
-/// See there for the details about fitting a TGraph.
+/// This will retrieve the function with name `fname` from ROOT's global list of functions, and use it to
+/// fit the data in the TGraph.
+/// TF1 or TF2 functions that have been created in the same ROOT session can be accessed using `fname`.
+/// Predefined functions such as gaus, expo, poln, etc. are automatically created by ROOT.
+/// @see TF1::InitStandardFunctions, TF2::InitStandardFunctions, TF3::InitStandardFunctions
 ///
-/// The parameter `fname` is the name of an already predefined function created by TF1 or TF2
-/// Predefined functions such as gaus, expo and poln are automatically
-/// created by ROOT.
+/// Note that using a global function is not thread safe. In this case, use the overload
+/// TGraph::Fit(TF1 *f1,Option_t *, Option_t *, Axis_t, Axis_t) with a locally created function.
+/// For more details about fitting a TGraph, see the same overload.
 ///
 /// The parameter `fname` can also be a formula, accepted by the linear fitter (linear parts divided
 /// by "++" sign), for example "x++sin(x)" for fitting "[0]*x+[1]*sin(x)"

hist/hist/src/TGraph2D.cxx

@@ -799,11 +799,19 @@ TObject *TGraph2D::FindObject(const TObject *obj) const
    return fFunctions ? fFunctions->FindObject(obj) : nullptr;
 }
 
-
 ////////////////////////////////////////////////////////////////////////////////
-/// Fits this graph with function with name fname
-/// Predefined functions such as gaus, expo and poln are automatically
-/// created by ROOT.
+/// Fit this graph with the global function named `fname`.
+///
+/// This will retrieve the function with name `fname` from ROOT's global list of functions, and use it to
+/// fit the data in the TGraph.
+/// TF1 or TF2 functions that have been created in the same ROOT session can be accessed using `fname`.
+/// Predefined functions such as gaus, expo and poln are automatically created by ROOT.
+///
+/// Note that using a global function is not thread safe. In this case, use the overload
+/// TGraph2D::Fit(TF2 *, Option_t *, Option_t *) with a locally created function.
+///
+/// For more details about fitting a TGraph, see TGraph::Fit().
+///
 /// fname can also be a formula, accepted by the linear fitter (linear parts divided
 /// by "++" sign), for example "x++sin(y)" for fitting "[0]*x+[1]*sin(y)"
 

hist/hist/src/TH1.cxx

@@ -3864,7 +3864,7 @@ TObject *TH1::FindObject(const TObject *obj) const
 ///
 ///
 /// fname is the name of a function available in the global ROOT list of functions
-/// `gROOT->GetListOfFunctions`
+/// `gROOT->GetListOfFunctions`. Note that this is not thread safe.
 /// The list include any TF1 object created by the user plus some pre-defined functions
 /// which are automatically created by ROOT the first time a pre-defined function is requested from `gROOT`
 /// (i.e. when calling `gROOT->GetFunction(const char *name)`).