8322751: ZGC: Fix comments about marking roots

Reviewed-by: eosterlund

Author: lgxbslgx

src/hotspot/share/gc/x/xHeap.cpp

@@ -249,7 +249,7 @@ void XHeap::mark_start() {
   // Enter mark phase
   XGlobalPhase = XPhaseMark;
 
-  // Reset marking information and mark roots
+  // Reset marking information
   _mark.start();
 
   // Update statistics

src/hotspot/share/gc/z/zGeneration.cpp

@@ -861,7 +861,7 @@ void ZGenerationYoung::mark_start() {
   // Enter mark phase
   set_phase(Phase::Mark);
 
-  // Reset marking information and mark roots
+  // Reset marking information
   _mark.start();
 
   // Flip remembered set bits
@@ -1213,7 +1213,7 @@ void ZGenerationOld::mark_start() {
   // Enter mark phase
   set_phase(Phase::Mark);
 
-  // Reset marking information and mark roots
+  // Reset marking information
   _mark.start();
 
   // Update statistics