Update steps in debugging instruction of CLR (#43557)

* Update CLR debugging instruction steps.

* Use suggested BCL path

Co-authored-by: Jan Vorlicek <janvorli@microsoft.com>

* Mention the usage cases of CORE_LIBRARIES.

Co-authored-by: Jan Vorlicek <janvorli@microsoft.com>

docs/workflow/debugging/coreclr/debugging.md

@@ -19,10 +19,18 @@ Debugging CoreCLR on Windows
 7. Set Command Arguments=`<managed app you wish to run>` (e.g. HelloWorld.dll)
 8. Set Working Directory=`$(SolutionDir)\..\..\..\bin\coreclr\Windows_NT.$(Platform).$(Configuration)`
     1. This points to the folder containing CoreCLR binaries.
-9. Press F11 to start debugging at wmain in corerun (or set a breakpoint in source and press F5 to run to it)
+9. Set Environment=`CORE_LIBRARIES=$(SolutionDir)\..\..\..\bin\runtime\<current tfm>-Windows_NT-$(Configuration)-$(Platform)`,
+    where '\<current tfm\>' is the target framework of current branch, for example `netcoreapp3.1` `net5.0`.
+    1. This points to the folder containing core libraries except `System.Private.CoreLib`.
+    2. This step can be skipped if you are debugging CLR tests that references only `System.Private.CoreLib`.
+    Otherwise, it's required to debug a realworld application that references anything else, including `System.Runtime`.
+10. Right-click the INSTALL project and choose 'Build'
+    1. This will load necessary information from cmake to Visual Studio.
+11. Press F11 to start debugging at wmain in corerun (or set a breakpoint in source and press F5 to run to it)
     1. As an example, set a breakpoint for the EEStartup function in ceemain.cpp to break into CoreCLR startup.
 
-Steps 1-8 only need to be done once, and then (9) can be repeated whenever you want to start debugging. The above can be done with Visual Studio 2013.
+Steps 1-10 only need to be done once, and then (11) can be repeated whenever you want to start debugging. The above can be done with Visual Studio 2019 as writing.
+Keeping with latest version of Visual Studio is recommended.
 
 ### Using SOS with windbg or cdb on Windows ###