Clarify the meaning of MaxDegreeOfParallelism = -1 (#7879)

Author: theodorzoulias

xml/System.Threading.Tasks/ParallelOptions.xml

@@ -170,7 +170,7 @@ By default, methods on the <xref:System.Threading.Tasks.Parallel> class attempt
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 ## Remarks  
- The <xref:System.Threading.Tasks.ParallelOptions.MaxDegreeOfParallelism%2A> property affects the number of concurrent operations run by <xref:System.Threading.Tasks.Parallel> method calls that are passed this <xref:System.Threading.Tasks.ParallelOptions> instance. A positive property value limits the number of concurrent operations to the set value. If it is -1,  there is no limit on the number of concurrently running operations.  
+ The <xref:System.Threading.Tasks.ParallelOptions.MaxDegreeOfParallelism%2A> property affects the number of concurrent operations run by <xref:System.Threading.Tasks.Parallel> method calls that are passed this <xref:System.Threading.Tasks.ParallelOptions> instance. A positive property value limits the number of concurrent operations to the set value. If it is -1,  there is no limit on the number of concurrently running operations (with the exception of the <xref:System.Threading.Tasks.Parallel.ForEachAsync%2A> method, where -1 means <xref:System.Environment.ProcessorCount%2A>).  
   
  By default, <xref:System.Threading.Tasks.Parallel.For%2A> and <xref:System.Threading.Tasks.Parallel.ForEach%2A> will utilize however many threads the underlying scheduler provides, so changing <xref:System.Threading.Tasks.ParallelOptions.MaxDegreeOfParallelism%2A> from the default only limits how many concurrent tasks will be used.