Table with list of default env vars per OS (#3291)

* Table with list of default env vars per OS

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update env variables list

* ADD windir env variable

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Bernát Gábor <[email protected]>

docs/changelog/2753.bugfix.rst

@@ -0,0 +1 @@
+Add table with default environment variables per OS

docs/config.rst

@@ -289,6 +289,155 @@ 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``.
 
+   Some variables are always passed through to ensure the basic functionality of standard library functions or tooling like
+   pip. This is also not case sensitive on all platforms except Windows.
+
+    .. list-table:: Environment Variables per Operating System
+        :widths: 25 25 25 25
+        :header-rows: 1
+
+        *   - Environment Variable
+            - Linux
+            - MacOS
+            - Windows
+        *   - https_proxy
+            - ✅
+            - ✅
+            - ✅
+        *   - http_proxy
+            - ✅
+            - ✅
+            - ✅
+        *   - no_proxy
+            - ✅
+            - ✅
+            - ✅
+        *   - LANG
+            - ✅
+            - ✅
+            - ✅
+        *   - LANGUAGE
+            - ✅
+            - ✅
+            - ✅
+        *   - CURL_CA_BUNDLE
+            - ✅
+            - ✅
+            - ✅
+        *   - SSL_CERT_FILE
+            - ✅
+            - ✅
+            - ✅
+        *   - CC
+            - ✅
+            - ✅
+            - ✅
+        *   - CFLAGS
+            - ✅
+            - ✅
+            - ✅
+        *   - CCSHARED
+            - ✅
+            - ✅
+            - ✅
+        *   - CXX
+            - ✅
+            - ✅
+            - ✅
+        *   - CPPFLAGS
+            - ✅
+            - ✅
+            - ✅
+        *   - LD_LIBRARY_PATH
+            - ✅
+            - ✅
+            - ✅
+        *   - LDFLAGS
+            - ✅
+            - ✅
+            - ✅
+        *   - HOME
+            - ✅
+            - ✅
+            - ✅
+        *   - FORCE_COLOR
+            - ✅
+            - ✅
+            - ✅
+        *   - NO_COLOR
+            - ✅
+            - ✅
+            - ✅
+        *   - TMPDIR
+            - ✅
+            - ✅
+            - ❌
+        *   - TEMP
+            - ❌
+            - ❌
+            - ✅
+        *   - TMP
+            - ❌
+            - ❌
+            - ✅
+        *   - USERPROFILE
+            - ❌
+            - ❌
+            - ✅
+        *   - PATHEXT
+            - ❌
+            - ❌
+            - ✅
+        *   - MSYSTEM
+            - ❌
+            - ❌
+            - ✅
+        *   - WINDIR
+            - ❌
+            - ❌
+            - ✅
+        *   - APPDATA
+            - ❌
+            - ❌
+            - ✅
+        *   - PROGRAMDATA
+            - ❌
+            - ❌
+            - ✅
+        *   - PROGRAMFILES(x86)
+            - ❌
+            - ❌
+            - ✅
+        *   - SYSTEMDRIVE
+            - ❌
+            - ❌
+            - ✅
+        *   - SYSTEMROOT
+            - ❌
+            - ❌
+            - ✅
+        *   - COMSPEC
+            - ❌
+            - ❌
+            - ✅
+        *   - PROCESSOR_ARCHITECTURE
+            - ❌
+            - ❌
+            - ✅
+        *   - NUMBER_OF_PROCESSORS
+            - ❌
+            - ❌
+            - ✅
+        *   - PIP_*
+            - ✅
+            - ✅
+            - ✅
+        *   - VIRTUALENV_*
+            - ✅
+            - ✅
+            - ✅
+
+
    More environment variable-related information
    can be found in :ref:`environment variable substitutions`.