Clarify that ProcessStartInfo.UseShellExecute does not mean bash (#436)

* Clarify that ProcessStartInfo.UseShellExecute does not mean bash

The property name is very confusing on unix-like platforms. Anyone
familiar with unix-like platforms will probably assume it means
/bin/bash or /bin/sh. Clarify that it does not mean that shell and that
it refers to the conecpt of shell on Windows, which is closer in meaning
to "desktop".

For an example of this tripping up users, see:
dotnet/core#1857. For more on the
cross-platform inconsistencies of UseShellExecute, see
https://github.com/dotnet/corefx/issues/24704

* Update ProcessStartInfo.xml

xml/System.Diagnostics/ProcessStartInfo.xml

@@ -1478,6 +1478,8 @@ p.WaitForExit();
 
 Setting this property to `false` enables you to redirect input, output, and error streams.
 
+The word "shell" in this context (`UseShellExecute`) refers to a graphical shell (similar to the Windows shell) rather than command shells (for example, `bash` or `sh`) and lets users launch graphical applications or open documents.
+
 > [!NOTE]
 > <xref:System.Diagnostics.ProcessStartInfo.UseShellExecute> must be `false` if the <xref:System.Diagnostics.ProcessStartInfo.UserName> property is not `null` or an empty string, or an <xref:System.InvalidOperationException> will be thrown when the <xref:System.Diagnostics.Process.Start(System.Diagnostics.ProcessStartInfo)?displayProperty=nameWithType> method is called.
 
@@ -1811,4 +1813,4 @@ When <xref:System.Diagnostics.ProcessStartInfo.UseShellExecute> is `false`, the
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>