update docs to mention ssh auth sock env var (#8523)

Closes #8358

Add some docs to alleviate confusion.

helper/communicator/config.go

@@ -113,7 +113,8 @@ type SSH struct {
 	// use this option with a key pair already configured in the source AMI,
 	// leave the `ssh_keypair_name` blank. To associate an existing key pair in
 	// AWS with the source instance, set the `ssh_keypair_name` field to the
-	// name of the key pair.
+	// name of the key pair. The environment variable `SSH_AUTH_SOCK` must be
+	// set for this option to work properly.
 	SSHAgentAuth bool `mapstructure:"ssh_agent_auth"`
 	// If true, SSH agent forwarding will be disabled. Defaults to `false`.
 	SSHDisableAgentForwarding bool `mapstructure:"ssh_disable_agent_forwarding"`

website/source/partials/helper/communicator/_SSH-not-required.html.md

@@ -46,7 +46,8 @@
     use this option with a key pair already configured in the source AMI,
     leave the `ssh_keypair_name` blank. To associate an existing key pair in
     AWS with the source instance, set the `ssh_keypair_name` field to the
-    name of the key pair.
+    name of the key pair. The environment variable `SSH_AUTH_SOCK` must be
+    set for this option to work properly.
     
 -   `ssh_disable_agent_forwarding` (bool) - If true, SSH agent forwarding will be disabled. Defaults to `false`.