Windows - guidance for shell mode in the REPL (#34633)

* Windows - guidance for shell mode in the REPL

The REPL shell mode is very non intuititive for basic Windows users: the mode does nothing  (like list environment variables, list directory) such a user expects, and throws errors as well. So I clarify the behavior, and show how to access one such Windows shell that most users understand under "shell mode".

* Update index.md

* Windows - guidance for shell mode in the REPL

As mentionned in my last comment: shell examples are now English speaking.
No other change.

stdlib/REPL/docs/src/index.md

@@ -135,6 +135,43 @@ julia> ; # upon typing ;, the prompt changes (in place) to: shell>
 shell> echo hello
 hello
 ```
+!!! note
+    For Windows users, Julia's shell mode does not expose windows shell commands.
+    Hence, this will fail:
+
+```julia-repl
+julia> ; # upon typing ;, the prompt changes (in place) to: shell>
+
+shell> dir
+ERROR: IOError: could not spawn `dir`: no such file or directory (ENOENT)
+Stacktrace!
+.......
+```
+However, you can get access to `PowerShell` like this:
+```julia-repl
+julia> ; # upon typing ;, the prompt changes (in place) to: shell>
+
+shell> powershell
+Windows PowerShell
+Copyright (C) Microsoft Corporation. All rights reserved.
+PS C:\Users\elm>
+```
+... and to `cmd.exe` like that (see the `dir` command):
+```julia-repl
+julia> ; # upon typing ;, the prompt changes (in place) to: shell>
+
+shell> cmd
+Microsoft Windows [version 10.0.17763.973]
+(c) 2018 Microsoft Corporation. All rights reserved.
+C:\Users\elm>dir
+ Volume in drive C has no label
+ Volume Serial Number is 1643-0CD7
+  Directory of C:\Users\elm
+
+29/01/2020  22:15    <DIR>          .
+29/01/2020  22:15    <DIR>          ..
+02/02/2020  08:06    <DIR>          .atom
+```
 
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