added find and modify to the documentation.

Author: craiggwilson

Docs/reference/content/reference/driver/crud/writing.md

@@ -88,4 +88,88 @@ var result = await collection.DeleteOneAsync(filter);
 // and
 
 var result = await collection.DeleteManyAsync(filter);
-```
+```
+
+## Find And Modify
+
+There are a certain class of operations using the [findAndModify command]({{< docsref "reference/command/findAndModify/" >}}) from the server. This command will perform some operation on the document and return the document either before or after the modification takes place. By default, the document is returned before the modification takes place.
+
+### FindOneAndDelete
+
+A single document can be deleted atomically using [`FindOneAndDeleteAsync`]({{< apiref "M_MongoDB_Driver_IMongoCollection_1_FindOneAndDeleteAsync__1" >}}).
+
+```csharp
+var filter = new BsonDocument("FirstName", "Jack");
+var result = await collection.FindOneAndDeleteAsync(filter);
+
+Assert(result["FirstName"] == "Jack"); // will be true if a document was found.
+```
+
+The above will find a document where the `FirstName` is `Jack` and delete it. It will then return the document that was just deleted.
+
+
+### FindOneAndReplace
+
+A single document can be replaced atomically using [`FindOneAndReplaceAsync`]({{< apiref "M_MongoDB_Driver_IMongoCollection_1_FindOneAndReplaceAsync__1" >}}).
+
+```csharp
+var filter = new BsonDocument("FirstName", "Jack");
+var replacementDocument = new BsonDocument("FirstName", "John");
+
+var result = await collection.FindOneAndReplaceAsync(filter, doc);
+
+Assert(result["FirstName"] == "Jack"); // will be true if a document was found.
+```
+
+The above will find a document where the `FirstName` is `Jack` and replace it with `replacementDocument`. It will then return the document that was replaced.
+
+### FindOneAndUpdate
+
+A single document can be updated atomically using [`FindOneAndUpdateAsync`]({{< apiref "M_MongoDB_Driver_IMongoCollection_1_FindOneAndUpdateAsync__1" >}}).
+
+```csharp
+var filter = new BsonDocument("FirstName", "Jack");
+var update = Builders<BsonDocument>.Update.Set("FirstName", "John");
+
+var result = await collection.FindOneAndUpdateAsync(filter, update);
+
+Assert(result["FirstName"] == "Jack"); // will be true if a document was found.
+```
+
+The above will find a document where the `FirstName` is `Jack` and set its `FirstName` field to `John`. It will then return the document that was replaced.
+
+
+### Options
+
+Each of the 3 operations has certain options available.
+
+#### ReturnDocument
+
+For Replacing and Updating, the [`ReturnDocument`]({{< apiref "T_MongoDB_Driver_ReturnDocument" >}}) enum can be provided. It allows you to return the document after the modification has taken place. The default is `Before`.
+
+For example:
+
+```csharp
+var filter = new BsonDocument("FirstName", "Jack");
+var update = Builders<BsonDocument>.Update.Set("FirstName", "John");
+
+var options = new FindOneAndUpdateOptions<BsonDocument, BsonDocument>
+{
+	ReturnDocument = ReturnDocument.After
+};
+var result = await collection.FindOneAndUpdateAsync(filter, update, options);
+
+Assert(result["FirstName"] == "John"); // will be true if a document was found.
+```
+
+#### Projection
+
+A projection can be provided to shape the result. The easiest way to build the projection is using a [projection builder]({{< relref "reference\driver\definitions.md#projections" >}}).
+
+#### Sort
+
+Since only a single document is selected, for filters that could result in multiple choices, a sort should be provided and the first document will be selected.
+
+#### IsUpsert
+
+For Replacing and Updating, `IsUpsert` can be specified such that, if the document isn't found, one will be inserted.