[DOC]: Merge and concatenate chapter for user guide

This adds a chapter on merge and concatenate to the Iris user guide. The chapter describes what merge and concatenate do, the differences between the two and hence the situation in which each should be used. The chapter also contains a section on issues commonly encountered with merge and concatenate and, where possible, provides the solution to avoid the problem occurring.
There is good reason for using the active voice - it "creates a more positive and emphatic style of writing."

Having said that, consistency is also important...