Use comment to clarify variable (dotnet#41776)

Author: adegeo

docs/core/install/includes/package-manager-failed-to-find-deb-new.md

@@ -7,7 +7,7 @@ ms.custom: linux-related-content
 ---
 
 ```bash
-# Define the OS version, name, and codename
+# Get OS version info which adds the $ID and $VERSION_ID variables
 source /etc/os-release
 
 # Download the Microsoft keys

docs/core/install/linux-ubuntu.md

@@ -192,7 +192,7 @@ Preview releases are **not** available in the Microsoft package repository. For
 [!INCLUDE [linux-prep-intro-apt](includes/linux-prep-intro-apt.md)]
 
 ```bash
-# Get OS version info
+# Get OS version info which adds the $ID and $VERSION_ID variables
 source /etc/os-release
 
 # Download Microsoft signing key and repository