chore: Document SSH passphrase injection as an experimental feature

Signed-off-by: Andrew Obuchowicz <aobuchow@redhat.com>

docs/additional-configuration.adoc

@@ -189,7 +189,7 @@ Prerequisites:
 ** The steps below assume the following environment variables are set:
 *** `$SSH_KEY`: path on disk to private key for SSH keypair (e.g. `~/.ssh/id_ed25519`)
 *** `$SSH_PUB_KEY`: path on disk to public key for SSH keypair (e.g. `~/.ssh/id_ed25519.pub`)
-*** `$PASSPHRASE`: SSH keypair passphrase (optional)
+*** `$PASSPHRASE`: SSH keypair passphrase (optional). *Note:* requires setting `config.enableExperimentalFeatures: true` in the DevWorkspaceOperatorConfig.
 *** `$NAMESPACE`: namespace where workspaces using the SSH keypair will be started.
 
 Process:
@@ -225,6 +225,8 @@ you must add the following in your `~/.bashrc`:
 ----
 [ -f $HOME/ssh-environment ] && source $HOME/ssh-environment
 ----
++
+*Note:*  Specifying a passphrase for an SSH key is an experimental feature and is controlled by the DevWorkspaceOperatorConfig's `config.enableExperimentalFeatures` field.
 
 3. Annotate the secret to configure automatic mounting to DevWorkspaces
 +