what is the difference between D and D/L flip-flops?
On the next page (2-19), they have a similar diagram, and the text describes the FF whose output is labeled Q0 thusly:


  In SDR mode, D0 feeds one of the flip-flops that then feeds the output. The flip-flop can be configured as a D-type register or latch.


So I would infer that in general, the label "D/L" denotes such a configurable FF.