[doc] more explanation for "what is FUZZER_LIB" part

Hi, when reading this part, [what is FUZZER_LIB](https://google.github.io/fuzzbench/getting-started/adding-a-new-fuzzer/#what-is-fuzzer_lib), it seems a bit hard to follow.

Maybe some more explanations needed for beginners, to distinguish their situations and know how to integrate their project in a more suitable way.

For example, the "persistent mode" differences.

Thanks.

cc: @jonathanmetzman