Add instructions/explanation for merging topics to MCP

One of the workflows for merging topics currently is as follows:

1. Go to topic
2. In quick mod tools select "merge topic"
3. Get redirected to modcp
4. Topics in the same forum are displayed, topics other than the current topic have "select for merging" before their name, and there is a forum navigation box above
5. Select the topic to merge into
6. Done

If the forum in which the original topic was contained just that one topic however, after selecting "merge topic" in quick mod tools there are no topics with the text "select for merging" next to them. There are also no instructions as to how to proceed. This is baffling to the extreme as the user essentially hits a brick wall.

The suggestion is to add instructions on top like other modcp pages do explaining to select a topic for merging in the list below, or use the forum selector to see topics in other forums.