[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

utils/prepare-code-coverage-artifact.py

@@ -6,6 +6,9 @@ from __future__ import print_function
 
 - Collate raw profiles into one indexed profile.
 - Generate html reports for the given binaries.
+
+Caution: The positional arguments to this script must be specified before any 
+optional arguments, such as --restrict.
 '''
 
 import argparse
@@ -84,7 +87,8 @@ if __name__ == '__main__':
                        help='Emit a unified report for all binaries')
     parser.add_argument('--restrict', metavar='R', type=str, nargs='*',
                        default=[],
-                       help='Restrict the reporting to the given source paths')
+                       help='Restrict the reporting to the given source paths'
+                   ' (must be specified after all other positional arguments)')
     args = parser.parse_args()
 
     if args.use_existing_profdata and args.only_merge: