docs: Clarify CustomConst::equal_consts (#1396)

This is just explaining existing behaviour, but I think it's important
to explain that we are not expecting the equivalent of `Eq` on a
structure that could be just an `f64`....

This does constrain impls to be symmetric and transitive, but all
current ones are, and I couldn't think of any... (indeed, "either Eq or
always return false" would describe all current impls, and this is still
significantly more permissive than that.)

hugr-core/src/ops/constant/custom.rs

@@ -75,8 +75,14 @@ pub trait CustomConst:
     /// Compare two constants for equality, using downcasting and comparing the definitions.
     ///
     /// If the type implements `PartialEq`, use [`downcast_equal_consts`] to compare the values.
+    ///
+    /// Note that this does not require any equivalent of [Eq]: it is permissible to return
+    /// `false` if in doubt, and in particular, there is no requirement for reflexivity
+    /// (i.e. `x.equal_consts(x)` can be `false`). However, we do expect both
+    /// symmetry (`x.equal_consts(y) == y.equal_consts(x)`) and transitivity
+    /// (if `x.equal_consts(y) && y.equal_consts(z)` then `x.equal_consts(z)`).
     fn equal_consts(&self, _other: &dyn CustomConst) -> bool {
-        // false unless overloaded
+        // false unless overridden
         false
     }