Documentation: Clarify discriminant requirements

[vmiquel-42]

The documentation of Language::discriminant says the following:

Type representing the cases of this language.
Used for short-circuiting the search for equivalent nodes.

Return the Discriminant of this node.

What properties must this discriminant uphold?

From a quick look at the code, I am inferring that it must:

• Be compatible with Language::matches: if a.matches(b) then a.discriminant() == b.discriminant() (and not necessarily the reciprocal)
• Be compatible with Ord: if a.discriminant() == c.discriminant() && a < b && b < c then a.discriminant() == b.discriminant()

Are there other requirements?