Relational: comments for part of the simplify code

I found this bit of code that uses `.equals(0)` confusing.  I think
I understand it now, maybe these comments would be useful for others.
In fact, most likely there is a bug somewhere.  If precision of dif is too low - we shouldn't get T/F.  But right now:
```
In [14]: e=sqrt(1 + sqrt(3)) + sqrt(3 + 3*sqrt(3)) - sqrt(10 + 6*sqrt(3))

In [15]: zz=e.evalf()

In [16]: zz
Out[16]: -0.e-122

In [17]: Eq(zz,0)
Out[17]: False

In [18]: zz._prec
Out[18]: 1
```

@smichr, looks as a bug ?