add docs for registering login handler

docs/source/api/accounts-multi.md

@@ -315,6 +315,15 @@ Accounts.setAdditionalFindUserOnExternalLogin(({serviceName, serviceData}) => {
   }
 })
 ```
+{% apibox "AccountsServer#registerLoginHandler" %}
+
+Use this to register your own custom authentication method. This is also used by all of the other inbuilt accounts packages to integrate with the accounts system.
+
+There can be multiple login handlers that are registered. When a login request is made, it will go through all these handlers to find its own handler.
+
+The registered handler callback is called with a single argument, the `options` object which comes from the login method. For example, if you want to login with a plaintext password, `options` could be `{ user: { username: <username> }, password: <password> }`,or `{ user: { email: <email> }, password: <password> }`.
+
+The login handler should return `undefined` if it's not going to handle the login request or else the login result object.
 
 <h2 id="accounts_rate_limit">Rate Limiting</h2>
 

packages/accounts-base/accounts_server.js

@@ -547,19 +547,14 @@ export class AccountsServer extends AccountsCommon {
   /// LOGIN HANDLERS
   ///
 
-  // The main entry point for auth packages to hook in to login.
-  //
-  // A login handler is a login method which can return `undefined` to
-  // indicate that the login request is not handled by this handler.
-  //
-  // @param name {String} Optional.  The service name, used by default
-  // if a specific service name isn't returned in the result.
-  //
-  // @param handler {Function} A function that receives an options object
-  // (as passed as an argument to the `login` method) and returns one of:
-  // - `undefined`, meaning don't handle;
-  // - a login method result object
-
+  /**
+   * @summary Registers a new login handler.
+   * @locus Server
+   * @param {String} [name] The type of login method like oauth, password, etc.
+   * @param {Function} handler A function that receives an options object
+   * (as passed as an argument to the `login` method) and returns one of
+   * `undefined`, meaning don't handle or a login method result object.
+   */
   registerLoginHandler(name, handler) {
     if (! handler) {
       handler = name;