Add a path to the generated Xcode workspace in the build document.

It was a bit unclear to me where the workspace is.

Review-Url: https://codereview.chromium.org/2894533002
Cr-Commit-Position: refs/heads/master@{#472689}

docs/ios/build_instructions.md

@@ -81,7 +81,8 @@ development and testing purposes.
 Since the iOS build is a bit more complicated than a desktop build, we provide
 `ios/build/tools/setup-gn.py`, which will create four appropriately configured
 build directories under `out` for Release and Debug device and simulator
-builds, and generates an appropriate Xcode workspace as well.
+builds, and generates an appropriate Xcode workspace
+(`out/build/all.xcworkspace`) as well.
 
 You can customize the build by editing the file `$HOME/.setup-gn` (create it if
 it does not exist).  Look at `src/ios/build/tools/setup-gn.config` for