fix: add documentation for many classes where it was missing

src/authentication/SimpleCredentialsExtractor.ts

@@ -2,6 +2,9 @@ import { Credentials } from './Credentials';
 import { CredentialsExtractor } from './CredentialsExtractor';
 import { HttpRequest } from '../server/HttpRequest';
 
+/**
+ * Credentials extractor which simply interprets the contents of the Authorization header as a webID.
+ */
 export class SimpleCredentialsExtractor extends CredentialsExtractor {
   public async canHandle(): Promise<void> {
     return undefined;