Document and add tests: collection conversions of mutable create a copy

[lrytz]

Follow-up for #11868.

converting a mutable collection (to(collection.mutable.X), to(collection.X), toArray) always creates a copy, even if the original collection has a conforming type. converting an immutable collection doesn't copy if the collection's type conforms to the result.

• https://github.com/scala/docs.scala-lang/blob/7daca3fc7ef4dfc047d9bbf96ca324b8a43c44b4/_overviews/collections-2.13/trait-iterable.md#L25 (for https://docs.scala-lang.org/overviews/collections-2.13/trait-iterable.html)
• https://github.com/scala/scala/blob/9090f2813e697a1092303ddbcef7b0783b393b63/src/library/scala/collection/IterableOnce.scala#L1246-L1289

[magnolia-k]

Made a PR about changes to the documentation.
The next step is to add tests, but I'd like some advice on which tests to add, or create a new test file.

[SethTisue]

@Ichoran would scala/scala-collection-laws be a better place than scala/scala for a test like this, cutting across many collection types?

[SethTisue]

the doc PR (scala/docs.scala-lang#1643) is now merged

[SethTisue]

ping @Ichoran about whether this is appropriate for scala-collection-laws?