message: add api docs for config loading functions
diff: @@ -1019,6 +1019,9 @@ impl Config {
  self.load_values_from(&self.cwd)
  }
  + /// Like [`load_values`](Config::load_values) but without merging config values.
  + ///
  + /// This is primarily crafted for `cargo config` command.
  pub(crate) fn load_values_unmerged(&self) -> CargoResult<Vec<ConfigValue>> {
  let mut result = Vec::new();
  let mut seen = HashSet::new();
  @@ -1035,6 +1038,9 @@ impl Config {
  Ok(result)
  }
  + /// Like [`load_includes`](Config::load_includes) but without merging config values.
  + ///
  + /// This is primarily crafted for `cargo config` command.
  fn load_unmerged_include(
  &self,
  cv: &mut CV,
  @@ -1054,6 +1060,7 @@ impl Config {
  Ok(())
  }
  + /// Start a config file discovery from a path and merges all config values found.
  fn load_values_from(&self, path: &Path) -> CargoResult<HashMap<String, ConfigValue>> {
  // This definition path is ignored, this is just a temporary container
  // representing the entire file.
  @@ -1075,10 +1082,22 @@ impl Config {
  }
  }
  + /// Loads a config value from a path.
  + ///
  + /// This is used during config file discovery.
  fn load_file(&self, path: &Path, includes: bool) -> CargoResult<ConfigValue> {
  self._load_file(path, &mut HashSet::new(), includes, WhyLoad::FileDiscovery)
  }
  + /// Loads a config value from a path with options.
  + ///
  + /// This is actual implementation of loading a config value from a path.
  + ///
  + /// * `includes` determines whether to load configs from [`config-include`].
  + /// * `seen` is used to check for cyclic includes.
  + /// * `why_load` tells why a config is being loaded.
  + ///
  + /// [`config-include`]: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#config-include
  fn _load_file(
  &self,
  path: &Path,
  @@ -1118,7 +1137,8 @@ impl Config {
  ///
  /// Returns `value` with the given include files merged into it.
  ///
  - /// `seen` is used to check for cyclic includes.
  + /// * `seen` is used to check for cyclic includes.
  + /// * `why_load` tells why a config is being loaded.
  fn load_includes(
  &self,
  mut value: CV,
  