better help, change priority of processing help request. (#15660)

tools/nnyeah/nnyeah/Errors.resx

@@ -38,7 +38,26 @@
 	</data>
 
 	<data name="N0007" xml:space="preserve">
-		<value>If you want to compare assemblies, the earlier and later assembly options *must* be set.</value>
+		<value>nnyeah is a tool for converting legacy Xamarin assemblies (pre .NET 6)
+to work with .NET 6 or later. It does this by scanning the --input assembly for
+dependencies of legacy methods, types, or fields and converts them to references
+in the --microsoft-assembly and writes the result in the --ouput assembly.
+nnyeah attempts to find the legacy Xamarin assembly automatically. If it can't
+find it, you will need to supply a path to it with the --xamarin-assembly
+argument.
+nnyeah needs to have a path to target Microsoft assembly using the
+--microsoft-assembly argument. If you do not have a specific version in mind,
+you can use this command for iOS:
+find $(dirname $(which dotnet)) -name Microsoft.iOS.dll -print | grep ref
+or this command for macOS:
+find $(dirname $(which dotnet)) -name Microsoft.macOS.dll -print | grep ref
+These commands may list more than one choice. You will probably want to use the
+one with the highest version.
+Example invocation for an assembly that originally targets Xamarin.iOS.dll:
+dotnet nnyeah --input=/path/to/AssemblyToConvert.dll \
+              --output=/path/to/Converted.dll \
+	      --microsoft-assembly=/path/to/Microsoft.iOS.dll
+		</value>
 	</data>
 
 	<data name="N0008" xml:space="preserve">

tools/nnyeah/nnyeah/Program.cs

@@ -44,6 +44,12 @@ static int Main2 (string [] args)
 				doHelp = true;
 			}
 
+			if (doHelp) {
+				options.WriteOptionDescriptions (Console.Out);
+				Console.Out.WriteLine (Errors.N0007);
+				return 0;
+			}
+
 			if (infile is null || outfile is null) {
 				throw new ConversionException (Errors.E0014);
 			}
@@ -53,14 +59,7 @@ static int Main2 (string [] args)
 				throw new ConversionException (Errors.E0015);
 			}
 
-			if (doHelp) {
-				options.WriteOptionDescriptions (Console.Out);
-				Console.Out.WriteLine (Errors.N0007);
-				return 0;
-			}
-			else {
-				return AssemblyConverter.Convert (xamarinAssembly, microsoftAssembly!, infile!, outfile!, verbose, forceOverwrite, suppressWarnings);
-			}
+			return AssemblyConverter.Convert (xamarinAssembly, microsoftAssembly!, infile!, outfile!, verbose, forceOverwrite, suppressWarnings);
 		}
 	}