Docs link of env var subs section to set_env (#3039)

* Hyperlinked env var substitutions docs in set_env

* Added link to pass_env and tweaked wording a bit

* Added changelog entry

* Using :ref: to preserve title name (avoid redefining name)

docs/changelog/3039.doc.rst

@@ -0,0 +1,2 @@
+Linked environment variable substitutions docs in
+``set_env`` and ``pass_env`` config docs.

docs/config.rst

@@ -285,6 +285,9 @@ Base options
    tox invocation environment it is ignored. The list of environment variable names is not case sensitive, for example:
    passing ``A`` or ``a`` will pass through both ``A`` and ``a``.
 
+   More environment variable-related information
+   can be found in :ref:`environment variable substitutions`.
+
 .. conf::
    :keys: set_env, setenv
 
@@ -300,6 +303,9 @@ Base options
        - each line is in KEY=VALUE format; both the key and the value are stripped,
        - there is no special handling of quotation marks, they are part of the key or value.
 
+   More environment variable-related information
+   can be found in :ref:`environment variable substitutions`.
+
 .. conf::
    :keys: parallel_show_output
    :default: False
@@ -830,6 +836,8 @@ Special substitutions that accept additional colon-delimited ``:`` parameters
 cannot have a space after the ``:`` at the beginning of line (e.g.  ``{posargs:
 magic}`` would be parsed as factorial ``{posargs``, having value magic).
 
+.. _`environment variable substitutions`:
+
 Environment variable substitutions
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~