Clarify default new-tab command in help

src/cascadia/LocalTests_TerminalApp/CommandlineTest.cpp

@@ -210,6 +210,9 @@ namespace TerminalAppLocalTests
                 appArgs._exitMessage.c_str()));
             VERIFY_ARE_EQUAL(0, result);
             VERIFY_ARE_NOT_EQUAL("", appArgs._exitMessage);
+            VERIFY_ARE_NOT_EQUAL(std::string::npos, appArgs._exitMessage.find("new-tab"));
+            VERIFY_ARE_NOT_EQUAL(std::string::npos, appArgs._exitMessage.find("wt new-tab --help"));
+            VERIFY_ARE_NOT_EQUAL(std::string::npos, appArgs._exitMessage.find("-d,--startingDirectory"));
         }
     }
 

src/cascadia/TerminalApp/AppCommandlineArgs.cpp

@@ -143,6 +143,7 @@ void AppCommandlineArgs::_buildParser()
     // E.g., for "wt.exe -M -d c:/", we will use -M for the launch mode, but once we will encounter -d
     // we will know that the prefix is over and try to handle the suffix as a new tab subcommand
     _app.prefix_command();
+    _app.footer(RS_A(L"CmdHelpFooter"));
 
     // -v,--version: Displays version info
     auto versionCallback = [this](int64_t /*count*/) {

src/cascadia/TerminalApp/Resources/en-US/Resources.resw

@@ -433,6 +433,10 @@
   <data name="CmdSizeDesc" xml:space="preserve">
     <value>Specify the number of columns and rows for the terminal, in "c,r" format.</value>
   </data>
+  <data name="CmdHelpFooter" xml:space="preserve">
+    <value>If no command is specified, "new-tab" is assumed by default. Use "wt new-tab --help" to view new-tab options, including "-d,--startingDirectory".</value>
+    <comment>{Locked="\"new-tab\"","\"wt new-tab --help\"","\"-d,--startingDirectory\""}</comment>
+  </data>
   <data name="NewTabSplitButton.[using:Windows.UI.Xaml.Automation]AutomationProperties.HelpText" xml:space="preserve">
     <value>Press the button to open a new terminal tab with your default profile. Open the flyout to select which profile you want to open.</value>
   </data>