Clarify Buffer.MemoryCopy behavior for overlapping buffers (#4462)

Use the same wording as documentation for memmove C API.

Fixes dotnet/runtime#6419 (comment)

Author: jkotas

xml/System/Buffer.xml

@@ -383,8 +383,8 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- This method copies `sourceBytesToCopy` bytes from the address specified by `source` to the address specified by `destination`. If the buffers overlap and the difference between `destination` minus `source` is less than `sourceBytesToCopy`, the source block is copied to the destination block in reverse order.  
-  
+ This method copies `sourceBytesToCopy` bytes from the address specified by `source` to the address specified by `destination`. If some regions of the source area and the destination overlap, the function ensures that the original source bytes in the overlapping region are copied before being overwritten.
+
  ]]></format>
         </remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
@@ -448,7 +448,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- This method copies `sourceBytesToCopy` bytes from the address specified by `source` to the address specified by `destination`. If the buffers overlap and the difference between `destination` minus `source` is less than `sourceBytesToCopy`, the source block is copied to the destination block in reverse order.  
+ This method copies `sourceBytesToCopy` bytes from the address specified by `source` to the address specified by `destination`. If some regions of the source area and the destination overlap, the function ensures that the original source bytes in the overlapping region are copied before being overwritten.
   
  ]]></format>
         </remarks>