doc: fix docs on MongoDbIdentifiable and describe client id generation

Author: alastairtree

README.md

@@ -14,7 +14,6 @@ dotnet add package JsonApiDotNetCore.MongoDb
 ### Models
 
 ```cs
-// MongoDbIdentifiable is just a utility base class, could use  IIdentifiable<TId> instead
 public sealed class Book : MongoDbIdentifiable
 {
     [Attr]
@@ -90,9 +89,9 @@ public class Startup
 
 ### Customise MongoDB persistence options and _id generation
 
-In addition to `MongoDbIdentifiable` your resource classes are free to use any of the MongoDB driver persistence options or inherit from their own base class.
+`MongoDbIdentifiable` has some sensible defaults for storing documents with _ids, but these need to be customised if you want client side or string based ids.
 
-For example, you could change the example above so that the `Book` resource has string IDs rather than object ids in the DB, but still have them generated server side:
+For example, you could change the example above so that the `Book` resource has string IDs rather than object ids in the DB, (so far still generated server side):
 
 ```cs
 public class Book : IIdentifiable<string>
@@ -113,25 +112,24 @@ public class Book : IIdentifiable<string>
 }
 ```
 
-Resources just need to inherit from the base `IIdentifiable<string>` interface from JsonApiDotNetCore (or the provided default `MongoDbIdentifiable`) and then just use any of usual [MongoDB Driver mapping code](https://mongodb.github.io/mongo-csharp-driver/2.12/reference/bson/mapping/).
-
-You could also achieve the exact same result using MongoDB `BsonClassMap` [rather than attributes](https://mongodb.github.io/mongo-csharp-driver/2.11/reference/bson/mapping/) so your `Book` does not need any MongoDB specific code like below.
+Resources properties can use any of usual [MongoDB Driver mapping code](https://mongodb.github.io/mongo-csharp-driver/2.12/reference/bson/mapping/) so to  achieve the same result without decorating your json:api resource classes use `BsonClassMap` [rather than attributes](https://mongodb.github.io/mongo-csharp-driver/2.11/reference/bson/mapping/):
 
 ```cs
     // in startup
     BsonClassMap.RegisterClassMap<Book>(cm =>
     {
         cm.AutoMap();
         cm.MapIdProperty(x => x.Id).SetIdGenerator(StringObjectIdGenerator.Instance);
+        cm.MapMember(c => c.Name).SetElementName("bookName");
         cm.UnmapMember(x=>x.StringId);
     });
 ```
 
-Using `StringObjectIdGenerator` above could then be combined with `AllowClientGeneratedIds` JsonApi setting in `Startup.ConfigureServices` so that IDs can be generated on the client, and will be auto-assigned server side if not provided providing a flexible string based id for the `Book` resource:
+The `StringObjectIdGenerator` above can then be combined with `AllowClientGeneratedIds` JsonApi setting in `Startup.ConfigureServices` so that string IDs can be generated on the client, but will be auto-assigned to random strings server side if not provided. This style of ids will be more familiar to developers used to no-sql style databases.
 
 ```cs
     services.AddJsonApi(options => {
-        // Allow us to POST books with already assigned IDs!
+        // Allow us to POST books with already assigned IDs
         options.AllowClientGeneratedIds = true;
     }, resources: builder =>
     {