[docs] GettingStarted.md: Do not suggest incremental builds for generating Xcode projects

Given that
* we build the minimum when generating Xcode projects
* this routine need not be repeated with every Ninja build
* incremental builds are sensitive to a variety of changes
it is more foolproof to suggest a clean build up front than to tuck it away in
a list of build failure troubleshooting hints.

Author: AnthonyLatsis

Diff for: docs/HowToGuides/GettingStarted.md

@@ -350,7 +350,7 @@ while retaining the option of building with Ninja on the command line.
 
 Assuming that you have already [built the toolchain via Ninja](#the-actual-build),
 several more steps are necessary to set up this environment:
-* Generate Xcode projects with `utils/build-script --release --swift-darwin-supported-archs "$(uname -m)" --xcode`.
+* Generate Xcode projects with `utils/build-script --release --swift-darwin-supported-archs "$(uname -m)" --xcode --clean`.
   This will first build a few LLVM files that are needed to configure the
   projects.
 * Create a new Xcode workspace.