Minor comment improvements in buffer-pool.h
Minor comment improvements in buffer-pool.h

Change-Id: I1212de7ed4e1f93767e00d1f3245b02bb88fa015
Reviewed-on: http://gerrit.cloudera.org:8080/7774
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Impala Public Jenkins

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala