[Fuchsia] Add workstation boot image support

Allow downloading the workstation image from Fuchsia GCS bucket.
Add support for starting up Fuchsia emulators with the workstation image.

Bug: 1254074
Change-Id: I6eed72ad5e52836acd85cb8ad3f032f040f11a15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3192230
Commit-Queue: Chong Gu <[email protected]>
Reviewed-by: David Dorwin <[email protected]>
Cr-Commit-Position: refs/heads/main@{#926482}
diff --git a/docs/fuchsia/build_instructions.md b/docs/fuchsia/build_instructions.md
index 532640ca..8376ece 100644
--- a/docs/fuchsia/build_instructions.md
+++ b/docs/fuchsia/build_instructions.md
@@ -88,6 +88,16 @@
 target_os = ['fuchsia']
 ```
 
+The Fuchsia boot images that are checked out is controlled by the
+`checkout_fuchsia_boot_images` variable. For instance, adding
+```
+"checkout_fuchsia_boot_images": "qemu.x64,workstation.qemu-x64-release",
+```
+to the `custom_vars` section of your `.gclient` file would allow you to check
+out both images.
+
+```
+
 You will then need to run:
 
 ```shell