website: clarify shutdown command [GH-2011]

website/source/docs/builders/virtualbox-iso.html.markdown

@@ -179,9 +179,11 @@ each category, the available options are alphabetized and described.
   By default this is "output-BUILDNAME" where "BUILDNAME" is the name
   of the build.
 
-* `shutdown_command` (string) - The command to use to gracefully shut down
-  the machine once all the provisioning is done. By default this is an empty
-  string, which tells Packer to just forcefully shut down the machine.
+* `shutdown_command` (string) - The command to use to gracefully shut down the machine once all
+  the provisioning is done. By default this is an empty string, which tells Packer to just
+  forcefully shut down the machine unless a shutdown command takes place inside script so this may
+  safely be omitted. If one or more scripts require a reboot it is suggested to leave this blank
+  since reboots may fail and specify the final shutdown command in your last script.
 
 * `shutdown_timeout` (string) - The amount of time to wait after executing
   the `shutdown_command` for the virtual machine to actually shut down.

website/source/docs/builders/virtualbox-ovf.html.markdown

@@ -154,9 +154,11 @@ each category, the available options are alphabetized and described.
   By default this is "output-BUILDNAME" where "BUILDNAME" is the name
   of the build.
 
-* `shutdown_command` (string) - The command to use to gracefully shut down
-  the machine once all the provisioning is done. By default this is an empty
-  string, which tells Packer to just forcefully shut down the machine.
+* `shutdown_command` (string) - The command to use to gracefully shut down the machine once all
+  the provisioning is done. By default this is an empty string, which tells Packer to just
+  forcefully shut down the machine unless a shutdown command takes place inside script so this may
+  safely be omitted. If one or more scripts require a reboot it is suggested to leave this blank
+  since reboots may fail and specify the final shutdown command in your last script.
 
 * `shutdown_timeout` (string) - The amount of time to wait after executing
   the `shutdown_command` for the virtual machine to actually shut down.

website/source/docs/builders/vmware-vmx.html.markdown

@@ -109,9 +109,11 @@ each category, the available options are alphabetized and described.
   By default this is "output-BUILDNAME" where "BUILDNAME" is the name
   of the build.
 
-* `shutdown_command` (string) - The command to use to gracefully shut down
-  the machine once all the provisioning is done. By default this is an empty
-  string, which tells Packer to just forcefully shut down the machine.
+* `shutdown_command` (string) - The command to use to gracefully shut down the machine once all
+  the provisioning is done. By default this is an empty string, which tells Packer to just
+  forcefully shut down the machine unless a shutdown command takes place inside script so this may
+  safely be omitted. If one or more scripts require a reboot it is suggested to leave this blank
+  since reboots may fail and specify the final shutdown command in your last script.
 
 * `shutdown_timeout` (string) - The amount of time to wait after executing
   the `shutdown_command` for the virtual machine to actually shut down.