[Doc] xpu backend requires running setvars.sh (#6393)

Author: rscohn2

docs/source/getting_started/xpu-installation.rst

@@ -40,12 +40,13 @@ Quick start using Dockerfile
 Build from source
 -----------------
 
-- First, install required driver and intel OneAPI 2024.1.
+- First, install required driver and intel OneAPI 2024.1 or later.
 
 - Second, install Python packages for vLLM XPU backend building:
 
 .. code-block:: console
 
+    $ source /opt/intel/oneapi/setvars.sh
     $ pip install --upgrade pip
     $ pip install -v -r requirements-xpu.txt