[mtouch] Document --optimize=remove-unsupported-il-for-bitcode in mtouch --help. (#5609)

tools/common/Driver.cs

@@ -148,10 +148,11 @@ namespace Xamarin.Bundler {
 					"    inline-dynamic-registration-supported: By default always enabled (requires the linker). Optimizes calls to Runtime.DynamicRegistrationSupported to be a constant value. Makes the app smaller, and slightly faster at runtime.\n" +
 					"    register-protocols: Remove unneeded metadata for protocol support. Makes the app smaller and reduces memory requirements.\n" +
 #if !MONOTOUCH
-					"    trim-architectures: Remove unneeded architectures from bundled native libraries. Makes the app smaller and is required for macOS App Store submissions.\n",
+					"    trim-architectures: Remove unneeded architectures from bundled native libraries. Makes the app smaller and is required for macOS App Store submissions.\n" +
 #else
-					"",
+					"    remove-unsupported-il-for-bitcode: Remove IL that is not supported when compiling to bitcode, and replace with a NotSupportedException.\n" +
 #endif
+					"",
 					(v) => {
 						app.Optimizations.Parse (v);
 					});