doc: add comment to explain why unsetting PIP_REQUIRE_VIRTUALENV is required.

multi_user/environment.py

@@ -54,6 +54,10 @@ def install_package(name, version):
     logging.info(f"installing {name} version...")
     env = os.environ
     if "PIP_REQUIRE_VIRTUALENV" in env:
+        # PIP_REQUIRE_VIRTUALENV is an env var to ensure pip cannot install packages outside a virtual env
+        # https://docs.python-guide.org/dev/pip-virtualenv/
+        # But since Blender's pip is outside of a virtual env, it can block our packages installation, so we unset the
+        # env var for the subprocess.
         env = os.environ.copy()
         del env["PIP_REQUIRE_VIRTUALENV"]
     subprocess.run([str(PYTHON_PATH), "-m", "pip", "install", f"{name}=={version}"], env=env)