[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.
Does anyone know..? The docstrings for the two methods (which, admittedly, I re-wrote when we had all the problems with users mis-using these methods) are circularly referring... 

However the point of this note is the same as the updates to their docstrings: to tell people what **not** to use them for!