Update incoming email docs to clarify key use

doc/development/emails.md

@@ -54,9 +54,12 @@ 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. 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 `@`).
+     # 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 must appear in the "user" part of the address (before the `@`). It can be omitted but some features,
+     # including Service Desk, may not work properly.
      address: "gitlab-incoming+%{key}@gmail.com"
 
      # Email account username