Usability: Improve text in aggregator

Self-explanatory.

aggregator.admin.inc reveals two terms for one idea, neither of which are ideal in my opinion: "news items" and "feed items". This issue needs to decide if there is a better term to use, and to make consistent whatever term we do use. This version of the patch changes all instances of "news items" to "feed items", as "feed items" is used in the description of the module. Feel free to criticize that choice, and fix it :D

Also:
- '#description' => t('For most aggregation tasks, the default settings are fine.'),
+ '#description' => t('The default settings are fine for most aggregation tasks.'),

I prefer the latter because it gets the message across first, which is relevant "for most aggregation tasks" and can therefore be bumped up to the first bit of the message.