Clarification of a section of the Plus algorithm documentation

[mantid-builder]

Original Reporter: Russell Taylor

A user said:
"I have ben using Plus to concatenate runs for a while. I just happened to look at the wiki pages and saw that it seems like it might end up merging different detectors together is this true?"

They were referring to the text:
"Event lists at each workspace index are concatenated to create the output event list at the same workspace index. Note that in some cases, these event lists might be from different detectors; this is not checked against and the event lists will be concatenated anyway."

In fact they were fine to use Plus and this text should be updated to avoid alarming people by making clear the (rare) circumstances where this might be the case, and clarify that if the operands have been processed in the same way then this should never be the case.

[mantid-builder]

This issue was originally trac ticket 8612