comment: add some context and information about virtual modules ... m…

…odule

packages/gatsby/src/utils/gatsby-webpack-virtual-modules.ts

@@ -1,5 +1,20 @@
 import VirtualModulesPlugin from "webpack-virtual-modules"
 
+/*
+ * This module allows creating virtual (in memory only) modules / files
+ * that webpack compilation can access without the need to write module
+ * body to actual filesystem.
+ *
+ * It's useful for intermediate artifacts that are not part of final builds,
+ * but are used in some way to generate final ones (for example `async-requires.js`).
+ *
+ * Using virtual modules allow us to avoid unnecessary I/O to write/read those modules,
+ * but more importantly using virtual modules give us immediate invalidation events
+ * in webpack watching mode (as opposed to debounced/delayed events when filesystem is used).
+ * Instant invalidation events make it much easier to work with various state transitions
+ * in response to external events that are happening while `gatsby develop` is running.
+ */
+
 interface IGatsbyWebpackVirtualModulesContext {
   writeModule: VirtualModulesPlugin["writeModule"]
 }