docs(testnet-node): Add comments on ENV vars

src/pages/understand-stacks/running-testnet-node.md

@@ -93,9 +93,10 @@ Clone this repository:
 git clone https://github.com/blockstack/stacks-blockchain.git; cd stacks-blockchain
 ```
 
-Optionally, set env vars for a specific version, commit hash and branch:
+Change the below values to reflect the version, branch, and git commit of the source code being built for accuracy:
 
 ```bash
+# The following values are just an example
 export STACKS_NODE_VERSION=2.0.9
 export GIT_BRANCH=master
 export GIT_COMMIT=e7f178b