Update main.py: junit --help

Elaborate on the information given in --help for junit, to make it more correct and comprehensive.

Author: wyattscarpenter

mypy/main.py

@@ -1165,12 +1165,12 @@ def add_invertible_flag(
 
     misc_group = parser.add_argument_group(title="Miscellaneous")
     misc_group.add_argument("--quickstart-file", help=argparse.SUPPRESS)
-    misc_group.add_argument("--junit-xml", help="Write junit.xml to the given file")
+    misc_group.add_argument("--junit-xml", metavar="JUNIT_XML_OUTPUT_FILE", help="Write a JUnit XML test result document with type checking results to the given file")
     imports_group.add_argument(
         "--junit-format",
         choices=["global", "per_file"],
         default="global",
-        help="If --junit-xml is set, specifies format. global: single test with all errors; per_file: one test entry per file with failures",
+        help="If --junit-xml is set, specifies format. global (default): single test with all errors; per_file: one test entry per file with failures",
     )
     misc_group.add_argument(
         "--find-occurrences",