docs: document DENO_INSTALL env var on installation page

runtime/getting_started/installation.md

@@ -14,7 +14,10 @@ macOS, Linux, and Windows, on both x64 and arm64 architectures.
 ## Download and install
 
 [deno_install](https://github.com/denoland/deno_install) provides convenience
-scripts to download and install the binary.
+scripts to download and install the binary. The shell and PowerShell install
+scripts place the `deno` executable in `$HOME/.deno/bin` (or `$Home\.deno\bin`
+on Windows) by default; see [Binary location](#binary-location) below to install
+Deno somewhere else.
 
 <deno-tabs group-id="operating-systems">
 <deno-tab value="linux" label="Linux">
@@ -197,7 +200,24 @@ in the following default location:
 | Windows       | `%USERPROFILE%\.deno\bin\deno.exe` |
 
 Override the install directory by setting the `DENO_INSTALL` environment
-variable before running the install script.
+variable before running the install script; the binary is then placed in
+`$DENO_INSTALL/bin`. On macOS and Linux, set it on the piped shell itself so the
+installer (not just `curl`) sees the variable:
+
+```shell
+curl -fsSL https://deno.land/install.sh | DENO_INSTALL=/opt/deno sh
+```
+
+On Windows, set `$env:DENO_INSTALL` before running the PowerShell installer:
+
+```powershell
+$env:DENO_INSTALL = "C:\deno"
+irm https://deno.land/install.ps1 | iex
+```
+
+Remember to add the new `bin` directory to your `PATH`. See the
+[`deno_install` README](https://github.com/denoland/deno_install) for the
+canonical behavior and other supported options.
 
 When installed via a package manager (Homebrew, Scoop, etc.), the binary
 location is managed by that package manager.
@@ -237,9 +257,8 @@ your `PATH` yet. To fix this:
 - Confirm the install directory is on your `PATH`. The shell install script
   defaults to `~/.deno/bin` on macOS and Linux; for npm-based installs, run
   `npm config get prefix` to find the directory containing the global `bin`.
-- If you customised the install location, the shell install script's install
-  root can be overridden with the `DENO_INSTALL` environment variable, in which
-  case the binary lives at `$DENO_INSTALL/bin/deno`.
+- If you customised the install location, the binary lives at
+  `$DENO_INSTALL/bin/deno` — see [Binary location](#binary-location).
 
 ## Updating