Docs: Customization example: log_level, single task

[jaredbeck]

Is it useful to have another example of customization here? log_level is a pretty common thing to configure, I imagine. It might be nice, also, to have an example of customizing just one task, as this seems to.

It seems that Rake::SprocketsTask.new is slightly easier to use than Sprockets::Rails::Task.new, but also maybe I'm doing something completely wrong. I mean, the chances I'm doing something wrong are actually quite high 😅