Add enum discriminats to the reference.

Fixes #15755

Author: steveklabnik

src/doc/reference.md

@@ -1413,6 +1413,27 @@ a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
 In this example, `Cat` is a _struct-like enum variant_,
 whereas `Dog` is simply called an enum variant.
 
+Enums have a discriminant. You can assign them explicitly:
+
+```
+enum Foo {
+    Bar = 123,
+}
+```
+
+If a discriminant isn't assigned, they start at zero, and add one for each
+variant, in order.
+
+You can cast an enum to get this value:
+
+```
+# enum Foo { Bar = 123 }
+let x = Foo::Bar as u32; // x is now 123u32
+```
+
+This only works as long as none of the variants have data attached. If
+it were `Bar(i32)`, this is disallowed.
+
 ### Constant items
 
 ```{.ebnf .gram}