book: clarify datalog/datafrog differences

Author: lqd

book/src/rules/loans.md

@@ -64,7 +64,7 @@ The datalog rules below are considered the "naive" implementation, as it compute
 
 Some trivial differences exist with the implementation:
 - the use of the `;` alternative operator in the rules
-- the use of worst-case optimal joins in the implementation
+- some API limitations about joins in the implementation, sometimes requiring intermediate steps per join (and these can sometimes be shared between different rules)
 
 ### Subsets between origins