briefly document constructors

Author: nikomatsakis

book/src/formality_core/constructors.md

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+# Constructors
+
+Unless you include `#[customize(constructors)]`, the `#[term]` macro automatically creates constructors as follows:
+
+- For a `struct`, defines a `new` method that takes an `impl Upcast<T>` for each of your fields.
+- For an `enum`, defines a method per variant (converted to snake-case).
+  - If the name of the variant is a Rust keyword like `Struct`, the method will be called `struct_`.