Incorrect example of narrowing from Collection to RandomAccesss

[elizarov]

From https://kotlinlang.org/docs/jvm-api-guidelines-backward-compatibility.html#don-t-make-return-types-narrower

For example, sometimes you realize that you need index access to the elements of your collection and want to change the return type from Collection to RandomAccess.

However, RandomAccess is a marker interface, not a subtype of Collection, so this is an incorrect example.

Also, the section name "Don't make return types narrower" leads reader to an implicit assumption that "making return types wider" is somehow a good thing. It clearly needs a clarification that narrowing, unlike widening, is mostly source-compatible, so it is tempting to do, yet should not be done.