Display env vars for pip options in docs

Ref pypa#8044

Author: ctg123

docs/pip_sphinxext.py

@@ -13,9 +13,20 @@
 from sphinx.application import Sphinx
 
 from pip._internal.cli import cmdoptions
+from pip._internal.configuration import _normalize_name
 from pip._internal.commands import commands_dict, create_command
 from pip._internal.req.req_file import SUPPORTED_OPTIONS
 
+def convert_cli_option_to_envvar(opt_name):
+    undashed_opt_name = _normalize_name(opt_name)
+    normalized_opt_name = undashed_opt_name.upper().replace("-", "_")
+    return f"PIP_{normalized_opt_name}"
+
+def convert_cli_opt_names_to_envvars(original_cli_opt_names):
+    return [
+        convert_cli_option_to_envvar(opt_name)
+        for opt_name in original_cli_opt_names
+    ]
 
 class PipNewsInclude(rst.Directive):
     required_arguments = 1
@@ -130,7 +141,9 @@ def _format_option(
         opt_help = option.help.replace("%default", str(option.default))
         # fix paths with sys.prefix
         opt_help = opt_help.replace(sys.prefix, "<sys.prefix>")
-        return [bookmark_line, "", line, "", "    " + opt_help, ""]
+        env_var_names = convert_cli_opt_names_to_envvars(option._long_opts)
+        env_var_names_src = ", ".join(f"``{env_var}``" for env_var in env_var_names)
+        return [bookmark_line, "", line, "", "    " + opt_help, "", f"    (environment variable: {env_var_names_src})", ""]
 
     def _format_options(
         self, options: Iterable[optparse.Option], cmd_name: Optional[str] = None