Added ignoreStackAndRegisters parameter to GC_step

doc/gc.txt

@@ -56,7 +56,7 @@ file as well). With this switch the GC supports the following operations:
 
 .. code-block:: nim
   proc GC_setMaxPause*(MaxPauseInUs: int)
-  proc GC_step*(us: int, strongAdvice = false)
+  proc GC_step*(us: int, strongAdvice, ignoreStackAndRegisters = false)
 
 The unit of the parameters ``MaxPauseInUs`` and ``us`` is microseconds.
 
@@ -75,7 +75,11 @@ These two procs are the two modus operandi of the realtime GC:
     This allows the GC to perform some work for up to ``us`` time. This is
     useful to call in a main loop to ensure the GC can do its work. To
     bind all GC activity to a ``GC_step`` call, deactivate the GC with
-    ``GC_disable`` at program startup.
+    ``GC_disable`` at program startup. Notice that you may ask GC to not
+    scan stack and registers for references via ``ignoreStackAndRegisters``
+    parameter. This may reduce the step time depending on the stack depth,
+    but use it only when you are sure that neither the stack nor the registers
+    contain unique references to objects that must be preserved.
 
 These procs provide a "best effort" realtime guarantee; in particular the
 cycle collector is not aware of deadlines yet. Deactivate it to get more