windows/get-*: add PowerShell-only notice (#7402)

pages/windows/get-childitem.md

@@ -1,6 +1,7 @@
 # Get-ChildItem
 
 > List items in a directory.
+> This command can only be used through PowerShell.
 > More information: <https://docs.microsoft.com/powershell/module/microsoft.powershell.management/get-childitem>.
 
 - List all non-hidden items in the current directory:

pages/windows/get-content.md

@@ -1,6 +1,7 @@
 # Get-Content
 
 > Get the content of the item at the specified location.
+> This command can only be used through PowerShell.
 > More information: <https://docs.microsoft.com/powershell/module/microsoft.powershell.management/get-content>.
 
 - Display the content of a file:

pages/windows/get-filehash.md

@@ -1,6 +1,7 @@
 # Get-FileHash
 
 > Calculate a hash for a file.
+> This command can only be used through PowerShell.
 > More information: <https://docs.microsoft.com/powershell/module/microsoft.powershell.utility/get-filehash>.
 
 - Calculate a hash for a specified file using the SHA256 algorithm:

pages/windows/get-history.md

@@ -1,6 +1,7 @@
 # Get-History
 
 > Display PowerShell command history.
+> This command can only be used through PowerShell.
 > More information: <https://docs.microsoft.com/powershell/module/microsoft.powershell.core/get-history>.
 
 - Display the commands history list with ID: