Documentation: Clarify a few subtleties

* Clarify accepted values of <LANG> suffix in CMAKE_COMPILER_IS_GNU<LANG>.

* Clarify fact that target property GENERATOR_FILE_NAME usually cannot be
  used at configure time.

* Clarify that enable_language() can only be used in global scope of
  top-level project using language.

* State that enable_language(... OPTIONAL) currently doesn't work.

* Document regular expression operator precedence.

1 files changed, 6 insertions, 1 deletions

diff --git a/Source/cmEnableLanguageCommand.h b/Source/cmEnableLanguageCommand.h
index e4bb2517a6..ee963f9201 100644
--- a/Source/cmEnableLanguageCommand.h
+++ b/Source/cmEnableLanguageCommand.h
@@ -65,7 +65,12 @@ public:
       "any of the extra variables that are created by the project command. "
       "Example languages are CXX, C, Fortran. "
       "If OPTIONAL is used, use the CMAKE_<languageName>_COMPILER_WORKS "
-      "variable to check whether the language has been enabled successfully.";
+      "variable to check whether the language has been enabled successfully."
+      "\n"
+      "This command must be called on file scope (not inside a function) and "
+      "the language enabled can only be used in the calling project or its "
+      "subdirectories added by add_subdirectory(). Also note that at present, "
+      "the OPTIONAL argument does not work.";
     }
 
   cmTypeMacro(cmEnableLanguageCommand, cmCommand);