Explain that default_scope also influences update_all

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.
Reword documentation for update_all

It now contains a carefully formulated reference to the "current relation" which might help clarify that the receiving will generate its own scope, escaping the need for explicitly referencing `default_scope` which is, after all, just another way of specifying a scope and nothing special.
Merge pull request #17682 from ReneB/docs/update_all

Explain that default_scope also influences update_all