Comment out the symmetry tactic notation

It doesn't actually work, so it's best to not confuse people.  Instead,
I've added a comment about what we'd like to do.
Merge branch 'master' of https://github.com/HoTT/HoTT
More properties on relations in Z
Defined zNat : nat -> Z and proved it is an embedding
Defined zAbs : Z -> nat and proved computation rules for simple cases [n, 0] and [0, n]
Small fix in fraction_field.v