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.
I like it, though I would put the "with details given" first, since it might otherwise be misread as "the filtering of the current relation with details given", although it is supposed to mean "Updates ... with details given".

So, how about:
```ruby
# Updates all records with details given, respecting the filtering of the current relation. This method constructs
# a single SQL UPDATE statement and sends it straight to the database. It does not instantiate the involved
# models and it does not trigger Active Record callbacks or validations. Values passed to `update_all` will not
# go through ActiveRecord's type-casting behavior. It should receive only values that can be passed as-is to
# the SQL database.
```
or

```ruby
# Updates all records in the current relation with details given. This method constructs a single SQL UPDATE
# statement and sends it straight to the database. It does not instantiate the involved models and it does not
# trigger Active Record callbacks or validations. Values passed to `update_all` will not go through
# ActiveRecord's type-casting behavior. It should receive only values that can be passed as-is to the SQL
# database.
```