Document the [name]_setattr and [object]_[name]_setattr hooks

The documentation mentions that handlers can override `setattr` with something else to do additional processing when setting a value. However it doesn't mention that there are hooks which look for `[name]_setattr` and `[object]_[name]_setattr` methods and use those instead of `setattr` if they exist.

Whether or not this is a good idea, we should document it because occasionally it is the simplest way to get something done.

Find the existing documentation that says handlers can override `setattr` and read its surrounding explanation first. Document the `[name]_setattr` and `[object]_[name]_setattr` hooks, including that they are used instead of `setattr` when present; done means the documented behavior matches the issue description.

Written by the indexing model from the issue text.