Correctly document custom Read the Docs logic

Author: pradyunsg

doc/en/conf.py

@@ -251,7 +251,8 @@
 issues_github_path = "pytest-dev/pytest"
 
 # -- Custom Read the Docs build configuration -----------------------------------------
-# (undocumented, relies on implementation details of Read the Docs' build)
+# https://docs.readthedocs.io/en/stable/reference/environment-variables.html#environment-variable-reference
+# https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#including-content-based-on-tags
 
 IS_RELEASE_ON_RTD = (
     os.getenv("READTHEDOCS", "False") == "True"