Add documentation for UNCHANGED_ and ZERO_

[jodavies]

This resolves #585

[jodavies]

I would like to add a test-case for this, but I don't think everything works quite as expected. Here, test3 becomes 0 in the 5th module, but the 6th module's print has test3 as both zero and unchanged, which is not correct.

#-

#procedure exprinfo
	#message Module `CMODULE_':
	#do e = {`activeexprnames_'}
		#if `ZERO_`e''
			#message zero `e': `ZERO_`e''
		#endif
		#if `UNCHANGED_`e''
			#message unchanged `e': `UNCHANGED_`e''
		#endif
	#enddo
	#if `ZERO_'
		#message All zero: `ZERO_'
	#endif
	#if `UNCHANGED_'
		#message All unchanged: `UNCHANGED_'
	#endif
	#message
#endprocedure


Off stats;

Symbol x,y;

Local test1 = x;
Local test2 = y;
Local test3 = 1;
.sort

#call exprinfo
Identify x = 0;
.sort

#call exprinfo
Identify y = 0;
.sort

#call exprinfo
.sort

#call exprinfo
Multiply 0;
.sort

#call exprinfo
Print;
.end

[coveralls]

coverage: 48.455% (-0.03%) from 48.483%
when pulling 4c8d47e on jodavies:zero-unchanged
into 1e8e3da on vermaseren:master.

[jodavies]

Maybe it is OK that UNCHANGED_test3 is not correct in this instance. In general UNCHANGED is not hugely "reliable" anyway; Identify f(x?) = f(x); or Multiply 1;, etc etc all count as "changes" in this context.