ios docs: Suggest clearing the build folders.

docs/howto/build-run.md

@@ -505,3 +505,16 @@ Optionally, reset iOS simulator:
 ```
 iOS Menu > Simulator > Reset Content and Settings…
 ```
+
+If you get other build failures on iOS and you haven't changed
+anything in the `ios` folder yourself, there might residue from a
+previous build interfering in this one. So, try cleaning the build
+folder. A different folder is used for command-line builds and builds
+through Xcode.
+- If building from the command line with `react-native run-ios`, run
+  `rm -rf ios/build`.
+- If building in Xcode, the build folder is at a path like
+  `~/Library/Developer/Xcode/DerivedData/ZulipMobile-diuocloqwafvdpeozujwphdrhalf`
+  (the hash at the end will be different) by default, but if it's not
+  there, you can find it in Xcode at File > Workspace Settings > Build
+  Location.

docs/howto/ios-tips.md

@@ -7,6 +7,11 @@ environment](build-run.md#main-steps) for Zulip Mobile.
 `react-native run-ios` will launch a new terminal with the React Native
 packager and open up the app in the iOS simulator.
 
+If the build fails and you haven't changed anything in the `ios`
+folder yourself, there might residue from a previous build interfering
+in this one. Run `rm -rf ios/build` and try again, or see our [build
+failure troubleshooting doc][build-run-troubleshoot].
+
 It will also launch a browser tab in Chrome with the React Native debugger.
 `console.log` statements in React Native will end up in the JS console on
 this tab.
@@ -41,6 +46,16 @@ obtain Apple Developer credentials that will allow you to sign the app.
 Register at https://developer.apple.com. Then use your Apple ID in Xcode
 and choose it as your `Signing > Team` for both ZulipMobile and ZulipMobileTests.
 
+If it's not your first time, and you haven't changed anything in the
+`ios` folder yourself, you might see build failures caused by
+interfering residue from a previous build. If you do, clear Xcode's
+build folder for ZulipMobile with `rm -rf`, or through Finder. The
+default path for the build folder is
+`~/Library/Developer/Xcode/DerivedData/ZulipMobile-diuocloqwafvdpeozujwphdrhalf`
+(the hash at the end will be different), but you can find it in Xcode
+at File > Workspace Settings > Build Location. If that doesn't fix it,
+see our [build failure troubleshooting doc][build-run-troubleshoot].
+
 ### Tips when running on your iOS device
 When you change the BundleIdentifier and Team (required in order to run on a device),
 it **will** modify your `.pbxproj` file, which you do **not** want unless you intend
@@ -57,3 +72,4 @@ unstaged. Later, you can always [squash that commit with other
 commits][fixing-commits], if appropriate.
 
 [fixing-commits]: https://zulip.readthedocs.io/en/latest/git/fixing-commits.html
+[build-run-troubleshoot]: build-run.md#troubleshooting