Proposed patch for schema docs

[Jason Spence]

I was puzzling over cust_main_invoice for a while, and decided to
write up the fruits of my labor.  I've attached a patch against
schema.html.

Ivan, please let me know if this information is accurate; I'll go
ahead and commit it if you give me the thumbs-up.

-- 
 - Jason

USENET would be a better laboratory is there were more labor and less oratory.
		-- Elizabeth Haley