perlfunc: document that sort is stable, remove 'use sort' references

[mauke]

No description provided.

[Grinnz]

It's definitely worth documenting that this is how it currently works, but do we want to provide a guarantee? I suppose at this point it would be just as difficult to change that either way.

[jkeenan]

It's definitely worth documenting that this is how it currently works, but do we want to provide a guarantee? I suppose at this point it would be just as difficult to change that either way.

The DESCRIPTION section of perldoc sort is quite emphatic that we're not changing how sort works:

    The default sort has been stable since v5.8.0, and given this consistent
    behaviour for almost two decades, everyone has come to assume stability.

    Stability will remain the default - hence there is no need for a pragma
    for code to opt into stability "just in case" this changes - it won't.

    We do not foresee going back to offering multiple implementations of
    general purpose sorting - hence there is no future need to offer a
    pragma to choose between them.
...
    We are not averse to changing the sort algorithm, but we don't see the
    benefit in offering the choice of two general purpose implementations.