sim: Fixup comments and constness in draining infrastructure
sim: Fixup comments and constness in draining infrastructure

Fix comments that got outdated by the draining rewrite. Also fixup
constness for methods in the querying drain state in the DrainManager.

Weil0ng/gem5,rallylee/gem5,qizenguf/MLC-STT,KuroeKurose/gem5,cancro7/gem5,briancoutinho0905/2dsampling,KuroeKurose/gem5,gem5/gem5,aclifton/cpeg853-gem5,gedare/gem5,aclifton/cpeg853-gem5,cancro7/gem5,cancro7/gem5,aclifton/cpeg853-gem5,sobercoder/gem5,gem5/gem5,qizenguf/MLC-STT,cancro7/gem5,rallylee/gem5,Weil0ng/gem5,gedare/gem5,KuroeKurose/gem5,gem5/gem5,qizenguf/MLC-STT,rjschof/gem5,rjschof/gem5,sobercoder/gem5,SanchayanMaity/gem5,gem5/gem5,gem5/gem5,Weil0ng/gem5,SanchayanMaity/gem5,SanchayanMaity/gem5,TUD-OS/gem5-dtu,HwisooSo/gemV-update,cancro7/gem5,aclifton/cpeg853-gem5,rallylee/gem5,rjschof/gem5,Weil0ng/gem5,briancoutinho0905/2dsampling,HwisooSo/gemV-update,cancro7/gem5,qizenguf/MLC-STT,KuroeKurose/gem5,sobercoder/gem5,qizenguf/MLC-STT,HwisooSo/gemV-update,gedare/gem5,briancoutinho0905/2dsampling,aclifton/cpeg853-gem5,rjschof/gem5,powerjg/gem5-ci-test,rallylee/gem5,rallylee/gem5,Weil0ng/gem5,briancoutinho0905/2dsampling,SanchayanMaity/gem5,sobercoder/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,briancoutinho0905/2dsampling,gem5/gem5,briancoutinho0905/2dsampling,TUD-OS/gem5-dtu,aclifton/cpeg853-gem5,powerjg/gem5-ci-test,rjschof/gem5,gedare/gem5,TUD-OS/gem5-dtu,HwisooSo/gemV-update,SanchayanMaity/gem5,KuroeKurose/gem5,gedare/gem5,gedare/gem5,qizenguf/MLC-STT,TUD-OS/gem5-dtu,HwisooSo/gemV-update,KuroeKurose/gem5,powerjg/gem5-ci-test,SanchayanMaity/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,cancro7/gem5,HwisooSo/gemV-update,gedare/gem5,Weil0ng/gem5,briancoutinho0905/2dsampling,powerjg/gem5-ci-test,Weil0ng/gem5,HwisooSo/gemV-update,rjschof/gem5,KuroeKurose/gem5,qizenguf/MLC-STT,rallylee/gem5,powerjg/gem5-ci-test,powerjg/gem5-ci-test,aclifton/cpeg853-gem5,SanchayanMaity/gem5,sobercoder/gem5,rjschof/gem5,rallylee/gem5,powerjg/gem5-ci-test