Sybase has several whitepapers (I would assume) on how it works. As an FYI, there are three schemes starting from 11.9.2 and up:
APL, DPL, DRL;
All Pages Lock(ed): both the data pages in the table and the index pages (if you have indexes) are locked on modification
DPL: Data-Page Locking (I think that's the acronym). Data pages are locked.
DRL: Data-Row locking; Data rows are locked.