📝 Document when temp_dir setting and substitutions appeared (#2175)

Co-authored-by: Jürgen Gmach <juergen.gmach@googlemail.com>

docs/config.rst

@@ -82,6 +82,8 @@ Global settings are defined under the ``tox`` section as:
 
 .. conf:: temp_dir ^ PATH ^ {toxworkdir}/.tmp
 
+   .. versionadded:: 3.5.0
+
    Directory where to put tox temporary files. For example: we create a hard link (if possible,
    otherwise new copy) in this directory for the project package. This ensures tox works correctly
    when having parallel runs (as each session will have its own copy of the project package - e.g.
@@ -704,6 +706,11 @@ Globally available substitutions
     the directory where virtual environments are created and sub directories
     for packaging reside.
 
+``{temp_dir}``
+    the directory where tox temporary files live.
+
+    .. versionadded:: 3.16.1
+
 ``{homedir}``
     the user-home directory path.