[ios] Add warning about opening project instead of workspace

Bug: 1091876
Change-Id: I2653ce2664690823bd2df117dd1adcb4df00a33a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2238120
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Misha Efimov <mef@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777000}

docs/ios/build_instructions.md

@@ -76,6 +76,11 @@ build directories under `out` for Release and Debug device and simulator
 builds, and generates an appropriate Xcode workspace
 (`out/build/all.xcworkspace`) as well.
 
+Warning: *do not open `out/build/products.xcodeproj` as this file does not
+configure the "Legacy Build System". If you do open this file, many things
+won't work properly (clicking on an error won't open the corresponding file,
+...).*
+
 More information about [developing with Xcode](xcode_tips.md).
 
 You can customize the build by editing the file `$HOME/.setup-gn` (create it if

docs/ios/xcode_tips.md

@@ -5,8 +5,9 @@ to develop Chrome for iOS.
 
 ## Background
 
-### .xcworkspace and .xcproject
+### .xcworkspace and .xcodeproj
 
+*   Open all.xcworkspace only.
 *   Generated from BUILD.gn files.
 *   Don't make changes to it as it is a "fake" representation of the project
     -   changes will not be committed
@@ -15,6 +16,10 @@ to develop Chrome for iOS.
 *   Added BUILD.gn files/`source_sets` need to be referenced from other ones as
     a `dep` in order for it to be shown in xcode.
 
+Warning: *do not open the `.xcodeproj` as this file does not configure the
+"Legacy Build System". If you do open this file, many things won't work
+properly (clicking on an error won't open the corresponding file, ...).*
+
 ### Adding new files
 
 *   Create new files with `tools/boilerplate.py`