docs(python): Document start_transaction args (#9850)

Link to the `start_transaction` API docs, which describe all the possible arguments to `start_transaction`.

Closes GH-5082

Author: szokeasaurusrex

docs/platforms/python/performance/instrumentation/custom-instrumentation/index.mdx

@@ -29,6 +29,8 @@ def eat_pizza(pizza):
             eat_slice(pizza.slices.pop())
 ```
 
+The [API reference](https://getsentry.github.io/sentry-python/api.html#sentry_sdk.api.start_transaction) documents `start_transaction` and all its parameters.
+
 ## Add Spans to a Transaction
 
 If you want to have more fine-grained performance monitoring, you can add child spans to your transaction, which can be done by either: