Document that fluent `findBy(…)` queries must return a result

[mp911de]

Based on spring-projects/spring-data-jpa#3294, Query by Example and Querydsl findBy(…) methods must return a result to prevent leaking the query object. Query objects are often associated with state that is only valid inside the findBy(…) method (e.g. AOP, transactions) and the query object is no longer valid outside of the method call.