docs for `sort` refer to `Base.copymutable`, but the latter is not in the manual

[nsajko]

Uses Base.copymutable to support immutable collections and iterables.

If Base.copymutable is an internal function, why does the sort doc refer to it? Should Base.copymutable be made public?

[nsajko]

FTR, if it does get made public, it probably needs a better doc string, because I'm not sure how is one supposed to use it and/or overload it by reading the doc string.

Also, FWIW, Base.copymutable is used already in StaticArrays.

[LilithHafner]

In 1.9 the docstring for sort did not include this sentence, so it's not too late to keep copymutable internal. That said, it seems like sensible functionality to make public.

Also, the new docstring for sort! is objectively false for tuples because copymutable returns a Vector when passed a tuple and sort(::NTuple) returns an NTuple.