fix setenv usages in documentation examples

fixes #1999

Co-authored-by: Jürgen Gmach <[email protected]>

Author: AndreyNautilus

docs/changelog/1999.doc.rst

@@ -0,0 +1,2 @@
+Update examples in the documentation to use ``setenv`` in the ``[testenv]`` sections, not wrongly in the ``[tox]`` main section.
+- by :user:`AndreyNautilus`

docs/example/basic.rst

@@ -172,15 +172,15 @@ This variable can be also set in ``tox.ini``:
 
 .. code-block:: ini
 
-    [tox]
+    [testenv]
     setenv =
         PIP_INDEX_URL = https://pypi.my-alternative-index.org
 
 Alternatively, a configuration where ``PIP_INDEX_URL`` could be overriden from environment:
 
 .. code-block:: ini
 
-    [tox]
+    [testenv]
     setenv =
         PIP_INDEX_URL = {env:PIP_INDEX_URL:https://pypi.my-alternative-index.org}
 
@@ -192,21 +192,25 @@ multiple PyPI servers, using ``PIP_EXTRA_INDEX_URL`` environment variable:
 
 .. code-block:: ini
 
-    [tox]
+    [testenv]
     setenv =
         PIP_EXTRA_INDEX_URL = https://mypypiserver.org
-
-    [testenv]
     deps =
         # docutils will be installed directly from PyPI
         docutils
         # mypackage missing at PyPI will be installed from custom PyPI URL
         mypackage
 
 This configuration will install ``docutils`` from the default
-Python PYPI server and will install the ``mypackage`` from
+Python PyPI server and will install the ``mypackage`` from
 our index server at ``https://mypypiserver.org`` URL.
 
+.. warning::
+
+  Using an extra PyPI index for installing private packages may cause security issues.
+  For example, if ``mypackage`` is registered with the default PyPI index, pip will install ``mypackage``
+  from the default PyPI index, not from the custom one.
+
 Further customizing installation
 ---------------------------------