Fixed documentation for MESOS-5405.
Fixed documentation for MESOS-5405.

As MESOS-5405 changes the fields in `Request` to optional, we need to
update the documentation.

Review: https://reviews.apache.org/r/48263/

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