Improve online documentation for global configuration files

[mpilgrem]

Motivation: https://github.com/commercialhaskell/stack/pull/6607#issuecomment-2156701803

It appears to me that the following logic (from Stack.Config, extracts) is not fully documented online:

-- | Get the location of the default user configuration file. If a file already
-- exists at the deprecated location, its location is returned. Otherwise, the
-- new location is returned.
getDefaultUserConfigPath :: ...
...
userConfigPath <- getDefaultUserConfigPath configRoot
  extraConfigs0 <- getExtraConfigs userConfigPath >>=
    mapM (\file -> loadConfigYaml (parseConfigMonoid (parent file)) file)
...

-- | Determine the extra config file locations which exist.
--
-- Returns most local first
getExtraConfigs :: HasTerm env
                => Path Abs File -- ^ use config path
                -> RIO env [Path Abs File]
getExtraConfigs userConfigPath = do
  defaultStackGlobalConfigPath <- getDefaultGlobalConfigPath
  liftIO $ do
    env <- getEnvironment
    mstackConfig <-
        maybe (pure Nothing) (fmap Just . parseAbsFile)
      $ lookup "STACK_CONFIG" env
    mstackGlobalConfig <-
        maybe (pure Nothing) (fmap Just . parseAbsFile)
      $ lookup "STACK_GLOBAL_CONFIG" env
    filterM doesFileExist
        $ fromMaybe userConfigPath mstackConfig
        : maybe [] pure (mstackGlobalConfig <|> defaultStackGlobalConfigPath)

Stack.Constants (extracts):

-- | Deprecated default global config path.
-- Note that this will be @Nothing@ on Windows, which is by design.
defaultGlobalConfigPathDeprecated :: Maybe (Path Abs File)
defaultGlobalConfigPathDeprecated = parseAbsFile "/etc/stack/config"

-- | Default global config path.
-- Normally, @getDefaultGlobalConfigPath@ should be used instead.
-- Note that this will be @Nothing@ on Windows, which is by design.
defaultGlobalConfigPath :: Maybe (Path Abs File)
defaultGlobalConfigPath = parseAbsFile "/etc/stack/config.yaml"