Update: We had a bit of a misunderstanding on the update text. As commenter Gardner points out, the restart and other options on the power button menu were in the SHIELD Tablet 5.0 update. The "SHIELD Power Control" menu, a section of the primary Android Settings menu that was included in KitKat but dropped from the 5.0 update, is back in 5.0.1. Sorry for the confusion.
Inside the SHIELD Power Control menu are basic settings for the Tegra K1 processor and a few screen options, as well as more standard Android settings like a sleep timer for the screen and Wi-Fi.
NVIDIA is certainly hanging on to its reputation for speedy device updates, at least when it comes to the company's flagship Tegra tablet. The SHIELD Tablet is receiving its first post-Android 5.0 update, to the 5.0.1 build that's only a couple of weeks old in AOSP. But the gaming company isn't content to add the few behind-the-scenes changes from Google, and has added a few new tricks of its own, including one highly requested feature from previous versions of Android.
The power menu, the little buttons for volume control and Airplane Mode that appear when you hold the power button, was stripped out of Lollipop devices like the Nexus 6 and Nexus 9. But NVIDIA has slipped it back into the SHIELD Tablet, the same as it was on KitKat (and still is on the original SHIELD Portable). NVIDIA's version also includes a reboot option, which isn't available in stock Android. Other changes in the 5.0.1 update include new options for the built-in Camera Awesome app, support for OpenGL 4.5 graphics, slightly improved localization, and the usual memory and performance improvements. The update notes specifically mention improved switching between apps.
As with all Lollipop builds, the update will fail if you've modified your tablet in any way, including root. I can personally confirm that the update is also being issued for the SHIELD Tablet LTE model. Check your Settings menu if you want the update fast - NVIDIA seems to send it to everyone at once, unlike most phone manufacturer and carrier updates.