Add more documentation for LANGUAGE property.

Make it clear that if you set the language on a file, it will be compiled.

Source/cmSourceFile.cxx

@@ -472,7 +472,9 @@ void cmSourceFile::DefineProperties(cmake *cm)
      "What programming language is the file.",
      "A property that can be set to indicate what programming language "
      "the source file is. If it is not set the language is determined "
-     "based on the file extension. Typical values are CXX C etc.");
+     "based on the file extension. Typical values are CXX C etc. Setting "
+     "this property for a file means this file will be compiled. "
+     "Do not set this for header or files that should not be compiled.");
 
   cm->DefineProperty
     ("LOCATION", cmProperty::SOURCE_FILE,