add docs to local Antora extensions

lib/antora/extensions/version-fix.js

@@ -1,5 +1,19 @@
 'use strict'
 
+/**
+ * The purpose of this extension is to fix invalid metadata saved to either antora.yml or gradle.properties in certain
+ * tags. This invalid metadata prevents Antora from classifying the component versions properly.
+ *
+ * This extension addresses with the following cases:
+ * 
+ * . the boolean value on the prerelease key is incorrectly quoted
+ * . the prerelease tag is set to true for a GA version
+ * . the value of the name key is empty
+ * . the value of the displayVersion key doesn't match the actual version
+ * . the -SNAPSHOT suffix is appended to the value of the version key instead of the value of the prerelease key
+ *
+ * This extension should be listed directly after @antora/collector-extension.
+ */
 module.exports.register = function () {
   this.once('contentAggregated', ({ contentAggregate }) => {
     contentAggregate.forEach((componentVersionBucket) => {