KeySet is searchable but absent from docs

[som-snytt]

Reproduction steps

Scala version: 2.13

keyset and genkeyset are searchable in docs but omitted.

https://www.scala-lang.org/api/current/scala/collection/MapOps.html#GenKeySetextendsAnyRef

Problem

I expect to see docs for protected classes intended for implementors.

If it's intended to hide protected classes, then they should not be searchable.

Similar for dotty scala/scala3#18946

Relatedly #11727

[som-snytt]

Duplicates #12670 and related