Document how to replace deprecated indexserver configuration directiv…

…e. (tox-dev#1976)

Co-authored-by: Gasper Zejn <zelo.zejn@gmail.com>

docs/config.rst

@@ -537,9 +537,11 @@ Complete list of settings that you can put into ``testenv*`` sections:
 
     .. versionadded:: 0.9
 
-    (DEPRECATED, will be removed in a future version) Multi-line ``name =
-    URL`` definitions of python package servers.  Dependencies can
-    specify using a specified index server through the
+    (DEPRECATED, will be removed in a future version) Use :conf:`setenv`
+    to configure PIP_INDEX_URL environment variable, see below.
+
+    Multi-line ``name = URL`` definitions of python package servers.
+    You can specify an alternative index server for dependencies by applying the
     ``:indexservername:depname`` pattern.  The ``default`` indexserver
     definition determines where unscoped dependencies and the sdist install
     installs from.  Example:
@@ -553,6 +555,17 @@ Complete list of settings that you can put into ``testenv*`` sections:
     will make tox install all dependencies from this PyPI index server
     (including when installing the project sdist package).
 
+    The recommended way to set a custom index server URL is to use :conf:`setenv`:
+
+    .. code-block:: ini
+
+        [testenv]
+        setenv =
+            PIP_INDEX_URL = {env:PIP_INDEX_URL:https://pypi.org/simple/}
+
+    This will ensure the desired index server gets used for virtual environment
+    creation and allow overriding the index server URL with an environment variable.
+
 .. conf:: envdir ^ PATH ^ {toxworkdir}/{envname}
 
     .. versionadded:: 1.5