Fix .postTerminator usage message (#542)

* Fix synopsis for .postTerminator parsing strategy

* Add test for .postTerminator usage message generation

Author: TiagoMaiaL

Sources/ArgumentParser/Usage/UsageGenerator.swift

@@ -116,6 +116,9 @@ extension ArgumentDefinition {
     if help.options.contains(.isOptional) {
       synopsis = "[\(synopsis)]"
     }
+    if parsingStrategy == .postTerminator {
+      synopsis = "-- \(synopsis)"
+    }
     return synopsis
   }
 }

Tests/ArgumentParserUnitTests/UsageGenerationTests.swift

@@ -217,4 +217,13 @@ extension UsageGenerationTests {
     _testSynopsis(N.self, expected: "example [--a] [--b]")
     _testSynopsis(N.self, visibility: .hidden, expected: "example [--a] [--b]")
   }
+  
+  struct O: ParsableArguments {
+    @Argument var a: String
+    @Argument(parsing: .postTerminator) var b: [String] = []
+  }
+  
+  func testSynopsisWithPostTerminatorParsingStrategy() {
+    _testSynopsis(O.self, expected: "example <a> -- [<b> ...]")
+  }
 }