Documenting new hooks with aggregator pluggable architecture patch

Cool, thanks. You can go ahead and commit this yourself if you have CVS access. I'm going to move to correct queue for CVS doc updates.

This update addresses the issues brought up in comment #21 on #303930: Pluggable architecture for aggregator.module . I repeat them here for convenience:

* The submitted documentation mention multiple times "an array of data items on $feed->items" but does not detail what keys should/could be in there and what are the values.
* Still, the documentation, is aggregator_parse_info mandatory or optional? Same for the processor.

What's missing now is documentation for

* hook_aggregator_fetch()
* hook_aggregator_fetch_info()

Also missing:

* document data format of $feed->items[n]

As feed items are exposed through the API now, we need to document the expected data format.

* Added documentation of hook_aggregator_fetch() and hook_aggregator_fetch_info()
* Improved explanation of aggregation stages fetching-parsing-processing
* Added @ingroup and @see references

Missing:

* likely we're going to add hook_aggregator_remove() to #303930: Pluggable architecture for aggregator.module

This documentation can go directly into #303930: Pluggable architecture for aggregator.module since there should be an aggregator.api.php to patch against.