Document how deletes files get generated & consider verifying if possible

[aelkiss]

@moseshll reports:

Part of the deletes file comes from Zephir and part from postZephir.pm. The two deletes get glommed together into the final product. There may be some research to verify this is happening correctly and see if that's even possible to do.

The local ones (from postZephir.pm) are things that are missing 974s or parts of 974s - in the bowels of postZephir.pm

We are also wondering what the implications of having delete records in the solr are, and whether some of that has been superceded by the redirects.