Google had already posted the factory image for the Android 5.1.1 update for the Nexus 7 (2012) WiFi model, but now it appears that they are rolling out the OTA to those with the 2012 Nexus 7. It's great that the factory image was available, however, having the OTA is even better. As it's simpler to update the device, and a much smaller download than downloading the full factory image. The update weighs in at about 9.6MB, so it should only take a minute or two to download the update, then you'll need to allow some time for it to install.
The Android 5.1.1 update is only available for a few devices. Like the 2012 and 2013 Nexus 7 WiFi models, Nexus 10, and even the Samsung Galaxy S6 Edge on T-Mobile. Which is actually really surprising. We should see it hitting more devices in the near future though. The Android 5.1.1 update is mostly a bug fixer, so it's still an important update. For those wondering about the build, it's LMY47V. The changelog on the OTA screen simply says "improves performance and stability and fixes bugs". So you may not find any new features in this update.
It's kind of surprising to see the Nexus 7 (2012) still getting updates, considering it's approaching 3 years old at this point. In fact, at Google I/O this year, it will be 3 years old. Which we don't typically see updates for devices that are that old, in fact sometimes we don't even see them for devices that are just a year or two old. So good on Google for continuing to update it, even with the trouble the NVIDIA Tegra 3 has caused them. The Nexus 7 (2012) was announced at Google I/O in 2012, and was the first Nexus tablet. It was a $199 tablet with an NVIDIA Tegra 3 inside, along with 1GB of RAM and 8GB of storage (the 16GB version was $249, later in the year they released the 32GB model at $249 and moved the 16GB model to $199 price tag). There was also no rear-facing camera. Which was fine, as tablet cameras usually aren't that good anyways. Since then, Google has made the Nexus 7 (2013) and the Nexus 9 as successors to the original Nexus 7 tablet.