Add examples using envvars for passwords in nuget.config (NuGet#2092)

Beef up examples and description of encrypted passwords only working on windows.

Author: Rob Relyea

docs/reference/nuget-config-file.md

@@ -131,8 +131,8 @@ Stores usernames and passwords for sources, typically specified with the `-usern
 | Key | Value |
 | --- | --- |
 | username | The user name for the source in plain text. |
-| password | The encrypted password for the source. |
-| cleartextpassword | The unencrypted password for the source. |
+| password | The encrypted password for the source. Encrypted passwords are only supported on Windows, and only can be decrypted when used on the same machine and via the same user as the original encryption. |
+| cleartextpassword | The unencrypted password for the source. Note: environment variables can be used for improved security. |
 
 **Example:**
 
@@ -151,6 +151,21 @@ In the config file, the `<packageSourceCredentials>` element contains child node
 </packageSourceCredentials>
 ```
 
+When using unencrypted passwords stored in an environment variable:
+
+```xml
+<packageSourceCredentials>
+    <Contoso>
+        <add key="Username" value="[email protected]" />
+        <add key="ClearTextPassword" value="%ContosoPassword%" />
+    </Contoso>
+    <Test_x0020_Source>
+        <add key="Username" value="user" />
+        <add key="ClearTextPassword" value="%TestSourcePassword%" />
+    </Test_x0020_Source>
+</packageSourceCredentials>
+```
+
 When using unencrypted passwords:
 
 ```xml