Be more specific about polymorphic return types

I no longer say "polymorphic" since it's a bit ambiguous here.

Author: camelid

library/std/src/primitive_docs.rs

@@ -220,10 +220,10 @@ mod prim_bool {}
 /// }
 /// ```
 ///
-/// The reason is that, in the first example, there are many possible types for `!` to coerce
-/// to, because the function's return value is polymorphic. However, in the second example, the
-/// other branch returns a `0` of type `u32`, which is a concrete type that `!` can be coerced to.
-/// See issue [#36375] for more information on this quirk of `!`.
+/// The reason is that, in the first example, there are many possible types that `!` could coerce
+/// to, because the function can return one of many concrete types. However, in the second example,
+/// the other branch returns a `0` of type `u32`, which is a concrete type that `!` can be coerced
+/// to. See issue [#36375] for more information on this quirk of `!`.
 ///
 /// [#36375]: https://github.com/rust-lang/rust/issues/36375
 ///