MESOS-2943: Add comment for explicit return type.
MESOS-2943: Add comment for explicit return type.

Review: https://reviews.apache.org/r/36267

jpeach/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,asamerh4/mesos,chhsia0/mesos,kaysoky/mesos,craimbert/mesos,andschwa/mesos,craimbert/mesos,gsantovena/mesos,verizonlabs/mesos,verizonlabs/mesos,dforsyth/mesos,asamerh4/mesos,jpeach/mesos,reneploetz/mesos,asamerh4/mesos,jpeach/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,yuquanshan/customizedMesos,dforsyth/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,gsantovena/mesos,neilconway/mesos,dforsyth/mesos,craimbert/mesos,zmalik/mesos,zmalik/mesos,reneploetz/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,yuquanshan/customizedMesos,jpeach/mesos,andschwa/mesos,jpeach/mesos,neilconway/mesos,jpeach/mesos,neilconway/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,Gilbert88/mesos,dforsyth/mesos,gsantovena/mesos,Gilbert88/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,abudnik/mesos,zmalik/mesos,reneploetz/mesos,jpeach/mesos,asamerh4/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,dforsyth/mesos,verizonlabs/mesos,verizonlabs/mesos,andschwa/mesos,yuquanshan/customizedMesos,asamerh4/mesos,neilconway/mesos,kaysoky/mesos,chhsia0/mesos,craimbert/mesos,zmalik/mesos,kaysoky/mesos,neilconway/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,zmalik/mesos,gsantovena/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,verizonlabs/mesos,chhsia0/mesos,verizonlabs/mesos,andschwa/mesos,yuquanshan/customizedMesos,abudnik/mesos,craimbert/mesos,yuquanshan/customizedMesos,andschwa/mesos,asamerh4/mesos,craimbert/mesos,neilconway/mesos,asamerh4/mesos,reneploetz/mesos,zmalik/mesos