Fix IsIntegralRing to return false for rings that are euclidean but not integral by changing IsUniqueFactorizationRing and IsEuclideanRing to not imply IsIntegralRing anymore

[fingolfin]

This is an updated version of PR #3976 by @olexandr-konovalov -- I tried to update that PR but could not, so I opened a new one.

This fixes #3975 and closes #3976.

The main change is that I did not drop the implication

InstallTrueMethod(IsEuclideanRing, IsZmodnZObjNonprimeCollection and
    IsWholeFamily and IsRing);

Instead I changed IsUniqueFactorizationRing (and hence IsEuclideanRing) to no longer imply IsIntegral. I also adjusted docstrings.

Just to avoid confusion, I'd like to point out that IsIntegral is a filter, so it never will give "correct" results in all cases -- it doesn't check the mathematical property but rather has a technical meaning. So IsIntegralRing(R) giving true should mean that the ring R is indeed an integral domain (only up to the existence of another bug, of course). While it giving false does not mean there are zero divisors, just that GAP cannot infer / prove that the ring is integral.

In the discussion on PR #3976, @hulpke suggested that if we do the changes that I now did here, then we should perhaps introduce new filters IsEuclideanDomain or IsEuclideanIntegralRing that have the old meaning (i.e. an alias for IsEuclideanRing and IsIntegralRing). In principle I could do that (though we probably should avoid the "Domain" variant -- while I like that name, unfortunately GAP chose to overload the term domain (and also category) with a meaning that differs from what it normally means in math, and I am concerned about this causing confusion). However I am not sure what I'd do with it? So far all methods I looked at and code I tried seem to work fine with the new meaning. So while I am not opposed, my suggestion would be to hold back on adding that until we find a concrete usecase? Of course if @hulpke or someone else already has something specific in mind right now, please let me know!

[ThomasBreuer]

The changes resolve the inconsistencies as far as I see.
It would be helpful to add distinguishing examples to the manual section on IsEuclideanRing, for example IsEuclideanRing( Integers mod 6 ); likewise for IsIntegralRing.

[hulpke]

There is nothing in that would obviously cause problems. Nor do I dare (and probably nobody else does so) to claim that it resolves all issues.
Let's try it out and if problems arise revisit later.