Add instructions re: "flag_sockpuppets" flag to Moderator Instructions

A new feature for automated identification of potentially malicious posts was recently enabled:

> If a new user replies to a topic from the same IP address as the user who started the topic, flag both of their posts
> as potential spam.

This system produce a new type of automated flag for the moderators to review. Flag review instructions are hereby added
for this flag type alongside the existing instructions for other automated flags in the "Moderator Instructions" asset
post.

Author: per1234

content/categories/staff/moderation/_topics/moderator-instructions/1.md

@@ -23,6 +23,7 @@ For an overview of the Discourse moderation system, see the **[Discourse Moderat
     - ["Needs Approval" flag for account](#needs-approval-flag-for-account)
     - ["Needs Approval" flag for post](#needs-approval-flag-for-post)
     - [Akismet flagged post](#akismet-flagged-post)
+    - ["flag_sockpuppets" flag](#flag_sockpuppets-flag)
 - [Guides](#guides)
   - [Deal with inappropriate behavior](#deal-with-inappropriate-behavior)
   - [Delete spam](#delete-spam)
@@ -209,6 +210,37 @@ This flag is created by `@system` when a post is detected as possibly spam by [A
 
 ---
 
+<a name="sockpuppets-flag"></a>
+
+#### ["flag_sockpuppets" flag](#sockpuppets-flag)
+
+This flag is created by `@system` when a new user created a topic, and another new user at the same IP address replied. The flagged post must be reviewed by a moderator.
+
+---
+
+⚠ Legitimate users may happen to be assigned the same IP address. Be sure to carefully evaluate the flagged posts and user accounts before taking any administrative action.
+
+---
+
+<a name="if-the-post-is-benign-2"></a>
+
+##### [If the post is benign](#if-the-post-is-benign-2)
+
+1. Click the **<kbd>No, restore post</kbd>** button.
+
+<a name="if-the-post-is-malicious-2"></a>
+
+##### [If the post is malicious](#if-the-post-is-malicious-2)
+
+1. Click the **<kbd>Yes ▾</kbd>** button.
+1. From the dropdown menu, click **Suspend User**. The **"Suspend User"** dialog will open.
+1. From the **Select a timeframe** dropdown menu, select **Forever**.
+1. In the **Suspension Reason** text field, enter "malicious use of multiple accounts".
+1. From the **"What would you like to do with the associated post?"** dropdown menu, select **Delete the post**.
+1. Click the **<kbd>:no_entry_sign: Suspend</kbd>** button.
+
+---
+
 <a name="guides"></a>
 
 ## [Guides](#guides)