Update comment to indicate why exclusive lease usage in transaction for lib/gitlab/counters/buffered_counter.rb is necessary

See original issue in #440368 (closed)

Problem: commit_increment! updates counters using an exclusive lease while in a counter_record.transaction

Ask: rework the logic to move the exclusive lease out of the transaction block if possible

Based on discussion in thread, we won't remove the exclusive lock, but will instead update the comment to explain why. #441530 (comment 1810432939)