Update sys.rst

Fixes python#102249

Author: impact27

Doc/library/sys.rst

@@ -173,7 +173,11 @@ always available.
 
    Call ``func(*args)``, while tracing is enabled.  The tracing state is saved,
    and restored afterwards.  This is intended to be called from a debugger from
-   a checkpoint, to recursively debug some other code.
+   a checkpoint, to recursively debug or profile some other code.
+   
+   Tracing is suspended while calling a tracing function set by
+   :func:`settrace` or :func:`setprofile` to avoid infinite recursion. 
+   :func:`call_tracing` enables explicit recursion of tracing function. 
 
 
 .. data:: copyright