stdlib, resources: Added warn msgs and commets.

- Added deprecated warnings to Workload and Abstract workload.

- Added comments to the classes changed.

Change-Id: I671daacf5ef455ea65103bd96aa442486142a486

configs/example/gem5_library/riscvmatched-fs.py

@@ -43,7 +43,6 @@ from gem5.utils.requires import requires
 from gem5.isas import ISA
 from gem5.simulate.simulator import Simulator
 from gem5.resources.workload import Workload
-from gem5.resources.resource import obtain_resource
 
 import argparse
 
@@ -77,7 +76,7 @@ board = RISCVMatchedBoard(
 # In the case where the `-i` flag is passed, we add the kernel argument
 # `init=/root/exit.sh`. This means the simulation will exit after the Linux
 # Kernel has booted.
-workload = obtain_resource("riscv-ubuntu-20.04-boot")
+workload = Workload("riscv-ubuntu-20.04-boot")
 kernel_args = board.get_default_kernel_args()
 if args.to_init:
     kernel_args.append("init=/root/exit.sh")