[nfc] fix wrong docu in default error handler and various docu fixes

core/base/src/TErrorDefaultHandler.cxx

@@ -95,10 +95,10 @@ static void DebugPrint(const char *fmt, ...)
 
 
 /// The default error handler function. It prints the message on stderr and
-/// if abort is set it aborts the application.  Replaces the minimal error handler
-/// of TError.h as part of the gROOT construction.  TError's minimal handler is put
+/// if abort is set it aborts the application. Replaces the minimal error handler
+/// of TError.h as part of the gROOT construction. TError's minimal handler is put
 /// back in place during the gROOT destruction.
-/// @note `abort()` is only called if `abort_bool` is `true` and `level < gErrorIgnoreLevel`
+/// @note `abort()` is only called if `abort_bool` is `true` and `level >= gErrorIgnoreLevel`
 void DefaultErrorHandler(Int_t level, Bool_t abort_bool, const char *location, const char *msg)
 {
    if (gErrorIgnoreLevel == kUnset) {

core/foundation/inc/TError.h

@@ -137,8 +137,8 @@ R__EXTERN const char *kCheckMsg;
          ::Warning("", kCheckMsg, _QUOTE_(e), __LINE__, __FILE__); \
    } while (false)
 
-R__EXTERN Int_t  gErrorIgnoreLevel;
-R__EXTERN Int_t  gErrorAbortLevel;
-R__EXTERN Bool_t gPrintViaErrorHandler;
+R__EXTERN Int_t  gErrorIgnoreLevel;    ///< errors with level below this value will be ignored. Default is `kUnset`.
+R__EXTERN Int_t  gErrorAbortLevel;     ///< non-ignored errors with level equal or above this value will call abort(). Default is `kSysError+1`.
+R__EXTERN Bool_t gPrintViaErrorHandler;///< If true, ROOT's `Printf` will print via the currently active ROOT error handler; if false (default), it will use the standard `printf`.
 
 #endif

graf2d/gpad/src/TPad.cxx

@@ -5063,7 +5063,8 @@ static Bool_t ContainsTImage(TList *li)
 ///  generated in some loop one needs to detect the special cases of first
 ///  and last page and then munge the argument to Print() accordingly.
 ///
-///  The "[" and "]" can be used instead of "(" and ")".
+///  The "[" and "]" can be used instead of "(" and ")" to open / close without
+///  actual printing.
 ///
 /// Example:
 /// ~~~ {.cpp}

math/mathcore/src/TKDTree.cxx

@@ -115,9 +115,13 @@ in the 2 subtrees as close as possible. The following section gives more details
     kdtree->SetData(2, datatree->GetV3());
     kdtree->Build();
     }
+\endcode
+
     NOTE, that this implementation of kd-tree doesn't support adding new points after the tree has been built
     Of course, it's not necessary to use TTree::Draw(). What is important, is to have data columnwise.
     An example with regular arrays:
+
+\code{.cpp}
     {
     Int_t npoints = 100000;
     Int_t ndim = 3;