[utils] coverage: Add help text about the --restrict flag (NFC)

Passing the --restrict flag to the coverage prep script before other
positional arguments is wrong, because it prevents the argparse module
from telling apart arguments to --restrict versus positional arguments.

Pointed out by Sean Callanan!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294616 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed