[Docs] Add `nvm help` documentation for `nvm exec`

nvm.sh

@@ -1285,7 +1285,8 @@ nvm() {
       echo '    --reinstall-packages-from=<version> When installing, reinstall packages installed in <node|iojs|node version number>'
       echo '  nvm uninstall <version>               Uninstall a version'
       echo '  nvm use [--silent] <version>          Modify PATH to use <version>. Uses .nvmrc if available'
-      echo '  nvm run <version> [<args>]            Run <version> with <args> as arguments. Uses .nvmrc if available for <version>'
+      echo '  nvm exec <version> [<command>]        Run <command> on <version>. Uses .nvmrc if available for <version>'
+      echo '  nvm run <version> [<args>]            Run `node` on <version> with <args> as arguments. Uses .nvmrc if available for <version>'
       echo '  nvm current                           Display currently activated version'
       echo '  nvm ls                                List installed versions'
       echo '  nvm ls <version>                      List versions matching a given description'