Improve code documentation in Rules.cpp

[Anders Markvardsen]

Documentation in Objects/Rules.cpp is not bad, but looking at this as part of coverity warning (CID=1075601) and ticket #9943, in this ticket improve document of Rules.cpp lines:

Clear up the mess and delete the rule that we have changes delete tmpA;

i.e. explain better than using 'clear up the mess' to give confidence to the reader that deleting tmpA is the right thing to do, also with reference to the Rules.cpp line:

tmpA=TreeComp.third; get base item

[Stuart Campbell]

This ticket has been transferred to github issue ​10843