[SYCL][HIP] Add SYCL_CLANG_EXTRA_FLAGS docs

Author: npmiller

sycl/doc/GetStartedGuide.md

@@ -502,6 +502,11 @@ skipped.
 If CUDA support has been built, it is tested only if there are CUDA devices
 available.
 
+If testing with HIP for AMD, the lit tests will use `gfx906` as the default
+architecture. It is possible to change it by adding
+`-Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=<target>` to the CMake
+variable `SYCL_CLANG_EXTRA_FLAGS`.
+
 #### Run DPC++ E2E test suite
 
 Follow instructions from the link below to build and run tests: