Add long help for "language" command

Author: kastiglione

lldb/source/Commands/CommandObjectLanguage.cpp

@@ -21,6 +21,18 @@ CommandObjectLanguage::CommandObjectLanguage(CommandInterpreter &interpreter)
           "language <language-name> <subcommand> [<subcommand-options>]") {
   // Let the LanguageRuntime populates this command with subcommands
   LanguageRuntime::InitializeCommands(this);
+  SetHelpLong(
+      R"(
+      Language specific subcommands may be used directly (without the `language
+      <language-name>` prefix), when stopped on a frame written in that
+      language. For example, from a C++ frame, users may run `demangle`
+      directly, instead of `language cplusplus demangle`.
+
+      Language specific subcommands are only available when the command name
+      cannot be misinterpreted. Take the `demangle` command for example, if a
+      Python command named `demangle-tree` were loaded, then the invocation
+      `demangle` would run `demangle-tree`, not `language cplusplus demangle`.
+      )");
 }
 
 CommandObjectLanguage::~CommandObjectLanguage() = default;