Update postcss.js documentation

build/helpers/postcss.js

@@ -11,6 +11,10 @@
  * @typedef {import('postcss').AcceptedPlugin} AcceptedPlugin
  */
 
+/**
+ * @typedef {import('postcss').Postcss} Postcss
+ */
+
 /**
  * @typedef {import('postcss').ProcessOptions} ProcessOptions
  */
@@ -38,6 +42,10 @@
  * @property {AutoprefixerOptions} autoprefixer - The `autoprefixer()` options.
  */
 
+/**
+ * @type {Postcss|undefined}        postcss      - The discovered PostCSS function.
+ * @type {AcceptedPlugin|undefined} autoprefixer - The discovered Autoprefixer function.
+ */
 let postcss, autoprefixer;
 
 try {
@@ -50,11 +58,21 @@ try {
     // do nothing
 }
 
+/**
+ * @type {boolean} Whether PostCSS was discovered (TRUE) or not (FALSE).
+ */
 const supportsPostCSS = (typeof postcss === 'function');
 
+/**
+ * @type {PluginList} A list of supported plugins.
+ */
 const pluginsList = [
     autoprefixer,
 ];
+
+/**
+ * @type {PluginMap} A map of supported plugins.
+ */
 const pluginsMap = {
     'autoprefixer': autoprefixer,
 };
@@ -65,7 +83,8 @@ const pluginsMap = {
  * @param {PluginCollection} pluginsListOrMap - A list or map of plugins.
  *     If a map of plugins, the plugin name looks up `options`.
  * @param {PostCSSOptions}   options - The PostCSS wrapper options.
- * @return {?Processor}
+ *
+ * @returns {Processor|null}
  */
 function createProcessor(pluginsListOrMap, options)
 {
@@ -84,7 +103,8 @@ function createProcessor(pluginsListOrMap, options)
  * @param {PluginCollection} pluginsListOrMap - A list or map of plugins.
  *     If a map of plugins, the plugin name looks up `options`.
  * @param {PostCSSOptions}   options - The PostCSS wrapper options.
- * @return {PluginList}
+ *
+ * @returns {PluginList}
  */
 function parsePlugins(pluginsListOrMap, options)
 {