Update docs for scala.reflect.api

[ashawley]

After scala/scala#7587 removed a bunch of deprecated stuff for 2.13, a lot of the api docs need to be updated.

For instance, the code example at https://github.com/scala/scala/blob/v2.13.0/src/reflect/scala/reflect/api/Constants.scala#L137-L187

There may be others.

[l-cdc]

I will look into this.

[l-cdc]

@ashawley Please review. Should I also create a PR to fix this example in docs?

[ashawley]

Someone else more qualified should review the changes to the reflection API in scala/scala#8796 . Hopefully, someone steps forward.

PS. Thanks for catching the bad reference I made to the PR. I fixed it in the description above.

[l-cdc]

@SethTisue Please review, thanks!

[SethTisue]

I've merged @lorenzocc's PR, which auto-closed this ticket.

However this ticket is pretty open-ended — it's the kind of ticket that if we leave it open, is liable to remain open forever because no one is going to be sure whether something was missed or not.

It's arguable, but I suggest we leave it closed. In general, there are many inadequacies in our documentation and we don't keep individual open tickets for them, though we're very happy to accept pull requests that address them.