Help: Allow command-line help of per-language and per-config variables

Extend commit 4e50320978 (Help: Allow help for arbitrary keywords,
2023-08-31, v3.28.0-rc1~31^2).

Source/cmDocumentation.cxx

@@ -747,6 +747,11 @@ bool cmDocumentation::PrintHelpOneVariable(std::ostream& os)
   if (this->PrintFiles(os, cmStrCat("variable/", vname))) {
     return true;
   }
+  std::string const generalized = GeneralizeKeyword(vname);
+  if (generalized != vname &&
+      this->PrintFiles(os, cmStrCat("variable/", generalized))) {
+    return true;
+  }
   // Argument was not a variable.  Complain.
   os << "Argument \"" << this->CurrentArgument
      << "\" to --help-variable is not a defined variable.  "

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

@@ -1266,6 +1266,7 @@ run_cmake(ProfilingTest)
 unset(RunCMake_TEST_OPTIONS)
 
 run_cmake_with_options(help-arbitrary "--help" "CMAKE_CXX_IGNORE_EXTENSIONS")
+run_cmake_with_options(help-variable-lang "--help-variable" "CMAKE_CXX_PVS_STUDIO")
 
 if (WIN32 OR DEFINED ENV{HOME})
   set(config_dir_test print-config-dir)

Tests/RunCMake/CommandLine/help-variable-lang-stdout.txt

@@ -0,0 +1,13 @@
+CMAKE_<LANG>_PVS_STUDIO
+-----------------------
+
+.. versionadded:: 4.3
+
+Default value for ``<LANG>_PVS_STUDIO`` target property
+when ``<LANG>`` is ``C`` or ``CXX``.
+
+This variable is used to initialize the property on each target as it is
+created.  For example:
+
+ set\(CMAKE_CXX_PVS_STUDIO pvs-studio-analyzer analyze -a "GA\\;OP"\)
+ add_executable\(foo foo\.cxx\)