Document group_replication_certification_loop_chunk_size and group_replication_certification_loop_sleep_time

As part of PS-8450, we added two new variables 

• group_replication_certification_loop_chunk_size
• group_replication_certification_loop_sleep_time

These variables need to be documented.

certification_loop_chunk_size is the size of the chunk that must be processed during the certifier garbage collection phase after which the client transactions will be allowed to interleave

certification_loop_sleep_time is the sleep time in microseconds the certifier garbage collection loop allows client transactions to interleave