Add support for MULTI command in Redis

This commit adds support for the MULTI command in Redis. The MULTI command starts a transaction, allowing further commands to be queued but not executed. The
implementation handles the MULTI command and returns "+OK\r\n". Queueing commands and executing a transaction will be covered in later stages. The commit also
includes updated tests and notes for the changes.

course-definition.yml

@@ -3301,7 +3301,27 @@ stages:
     description_md: |
       In this stage, you'll add support for the `MULTI` command.
 
-      **🚧 We're still working on instructions for this stage**. You can find notes on how the tester works below.
+      ### The MULTI command
+
+      The [MULTI](https://redis.io/docs/latest/commands/multi/) command starts a transaction.
+
+      After a `MULTI` command is executed, any further commands from the same connection will be "queued" but not executed.
+
+      Example usage:
+
+      ```bash
+      $ redis-cli
+      > MULTI
+      OK
+      > SET foo 41
+      QUEUED
+      > INCR foo
+      QUEUED
+      ```
+
+      The queued commands can be executed using [EXEC](https://redis.io/docs/latest/commands/exec/), which we'll cover in later stages.
+
+      In this stage, you'll just add support for handling the `MULTI` command and returning `+OK\r\n`. We'll get to queueing commands in later stages.
 
       ### Tests
 
@@ -3311,13 +3331,17 @@ stages:
       ./spawn_redis_server.sh
       ```
 
-      The tester will then connect to your server as a Redis client and run the following commands:
+      The tester will then connect to your server as a Redis client and run the following command:
 
       ```bash
       $ redis-cli MULTI
       ```
 
       The tester will expect `+OK\r\n` as the response.
+
+      ### Notes
+
+      - We'll test queueing commands & executing a transaction in later stages.
     marketing_md: |
       In this stage, you'll implement the MULTI command.