Improve documentation around enabling incoming emails in gitlab.yml

doc/development/emails.md

@@ -4,7 +4,7 @@ group: unassigned
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 
-# Dealing with email in development
+# Working with email in development
 
 ## Ensuring compatibility with mailer Sidekiq jobs
 
@@ -54,7 +54,8 @@ See the [Rails guides](https://guides.rubyonrails.org/action_mailer_basics.html#
    incoming_email:
      enabled: true
 
-     # The email address including the `%{key}` placeholder that will be replaced to reference the item being replied to.
+     # The email address including the %{key} placeholder that will be replaced to reference the item being replied to. This %{key} should be included in its entirety within the email address and not replaced by another value.
+     # For example: emailadress+%key@gmail.com.
      # The placeholder can be omitted but if present, it must appear in the "user" part of the address (before the `@`).
      address: "gitlab-incoming+%{key}@gmail.com"