Fix discriminator propertyName inference (#1342)

* Fix discriminator propertyName inference

* Update docs

* Improve tests, fix additional bugs

Author: drwpow

.changeset/friendly-numbers-tease.md

@@ -0,0 +1,5 @@
+---
+"openapi-typescript": patch
+---
+
+Fix discriminator propertyName inference

docs/src/content/docs/advanced.md

@@ -488,4 +488,6 @@ Pet: components["schemas"]["Cat"] | components["schemas"]["Dog"] | components["s
 Cat: { type?: "cat"; } & components["schemas"]["PetCommonProperties"];
 ```
 
+_Note: you optionally could provide `discriminator.propertyName: "type"` on `Pet` ([docs](https://spec.openapis.org/oas/v3.1.0#discriminator-object)) to automatically generate the `type` key, but is less explicit._
+
 While the schema permits you to use composition in any way you like, it’s good to always take a look at the generated types and see if there’s a simpler way to express your unions & intersections. Limiting the use of `oneOf` is not the only way to do that, but often yields the greatest benefits.