Update comment.

Craig Ringer, reviewed by me.

Author: robertmhaas

src/backend/storage/ipc/procarray.c

@@ -1493,7 +1493,8 @@ GetMaxSnapshotSubxidCount(void)
  *			older than this are known not running any more.
  *		RecentGlobalXmin: the global xmin (oldest TransactionXmin across all
  *			running transactions, except those running LAZY VACUUM).  This is
- *			the same computation done by GetOldestXmin(true, true).
+ *			the same computation done by
+ *			GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM).
  *		RecentGlobalDataXmin: the global xmin for non-catalog tables
  *			>= RecentGlobalXmin
  *