Docs for new Update features (#2950)

Docs for new Updates features

apps/zui/docs/Installation.md

@@ -21,12 +21,8 @@ Once installed, no additional configuration is necessary. For your reference,
 a separate article describes the [filesystem paths](./support/Filesystem-Paths.md) where unpacked Zui
 binaries and saved user data are stored on each platform.
 
-Details are provided in each section below about when Zui may "auto-update" to
-newer releases and when manual upgrades are necessary. In all cases,
-updating to a newer version of the app will preserve your existing
-[user data](./support/Filesystem-Paths.md#user-data). Because the way user data
-is stored sometimes changes in newer releases, downgrades to older releases are
-_not_ currently supported.
+Zui checks over the network for newer releases that may be available for
+[update](#updates).
 
 If you run into any problems, you may want to browse the 
 [troubleshooting docs](./support/Troubleshooting.md). If you get stuck, [join our public Slack](https://www.brimdata.io/join-slack/)
@@ -39,25 +35,21 @@ and we'll be happy to help.
 * Choose between installing for only the current user (default) or a machine-wide install
 * Click **Finish** and Zui will launch when installation completes
 * Click the Zui icon on the Desktop or Start menu to relaunch in the future
+* Zui checks over the network for newer releases that may be available for [update](#updates)
 
 ![Windows Installation](media/Windows-installation.gif)
 
-As newer Windows releases are published, Zui will automatically download them
-and pop up a notification offering to restart the app to apply the update.
-
 ## macOS Installation
 
 * Download the Zui installer via the **macOS** link at the [Zui download](https://www.brimdata.io/download/) page
 * Launch the downloaded `.dmg` file to begin installation
 * Click and drag the Zui icon into the Applications folder
 * Click the Zui icon in the Applications folder to start Zui now and in the future
 * You may need to click through an "Are you sure?" prompt the first time Zui is launched
+* Zui checks over the network for newer releases that may be available for [update](#updates)
 
 ![macOS Installation](media/macOS-installation.gif)
 
-As newer macOS releases are published, Zui will automatically download them
-and pop up a notification offering to restart the app to apply the update.
-
 ## Linux Installation
 
 * Uninstall any currently-installed Zui release, using a tool like `yum`,
@@ -72,11 +64,43 @@ and pop up a notification offering to restart the app to apply the update.
 * Open the downloaded `.deb` or `.rpm` file in the Software Install utility and click **Install**
 * Enter administration credentials, if prompted
 * Click the Zui icon in the applications menu to start the app now and in the future
+* Zui checks over the network for newer releases that may be available for [update](#updates)
 
 ![Linux Installation](media/Linux-installation.gif)
 
-As newer Linux releases are published, Zui will pop up a notification of their
-availability, but the Linux releases do _not_ auto-update. Follow the link in
-the pop-up to the [Zui download](https://www.brimdata.io/download/) page to
-get the latest release. After uninstalling the prior version, install the new
-release using the appropriate package manager for your distribution.
+## Updates
+
+In a default configuration, each time Zui is launched it will check over the
+network for the availability of a newer release. If one is found, a
+notification like the following will pop up.
+
+![Update Available](media/Update-Available.png)
+
+If **Later** is clicked, the notification is dismissed and no update is
+performed. If **Install** is clicked, the result on each platform is as follows:
+
+|**Platform**|**Result**|
+|-|-|
+|**Windows**|An interactive wizard for the newer release will launch, just like the one from initial [installation](#windows-installation).|
+|**macOS**|The newer release will install in the background and Zui will automatically relaunch into the newer version.|
+|**Linux**|The [Zui download](https://www.brimdata.io/download/) page will open in your browser. Manual uninstall/reinstall is necessary on Linux, so repeat the same download/uninstall/reinstall steps as during initial [installation](#linux-installation).|
+
+In all cases, updating to a newer version of the app will preserve your existing
+[user data](./support/Filesystem-Paths.md#user-data). Because the way user data
+is stored can sometimes change in newer releases, downgrading to older releases is
+_not_ currently supported.
+
+Configurable **Settings** are available to change notification frequency.
+
+![Updates Settings](media/Updates-Settings.png)
+
+|**Setting**|**Result**|
+|-|-|
+|**On Startup** (default)|A check is performed for a newer release once each time Zui is launched|
+|**On Startup & Daily**|A check is performed for a newer release when Zui is launched and every 24 hours thereafter that the app remains open|
+|**Manually**|Zui will never pop up an automatic notification about a newer release|
+
+Regardless of settings, manually clicking the pull-down menu option
+**Help > Check for Updates** on Windows/Linux or **Zui > Check for Updates**
+on macOS will perform an immediate check and a notification will pop up if a newer
+release is detected.