Fixes #115525.

On [AbstractNode.detach] and its two progeny [RenderNode.detach]
and [Layer.detach], the docs said both to call the inherited method
before detaching children, and to end by doing so.  The former
advice is what's enforced by an assertion in the base implementation,
so cut out the other.

The corresponding [attach] methods redundantly said twice to
call the inherited method first, so cut the redundancy.
Leave in place the version more recently added (in #76021), because
that PR shows the old version must have been easy to overlook.