[CommandLine] Fix Missing Description for cl::values in Tutorial Document

[mshockwave]

Summary

In the command line library, cl::values should be end by null, or clEnumValEnd. However, the tutorial page for it doesn't follow the rule.
Example codes are fixed and some description text is added in this patch.

docs/CommandLine.rst

enum OptLevel {
     g, O1, O2, O3
   };
 
   cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
     cl::values(
       clEnumVal(g , "No optimizations, enable debugging"),
       clEnumVal(O1, "Enable trivial optimizations"),
       clEnumVal(O2, "Enable default optimizations"),
-      clEnumVal(O3, "Enable expensive optimizations")));
+      clEnumVal(O3, "Enable expensive optimizations"), clEnumValEnd));
 
   ...
     if (OptimizationLevel >= O2) doPartialRedundancyElimination(...);
   ...
 
 This declaration defines a variable "``OptimizationLevel``" of the
 "``OptLevel``" enum type.  This variable can be assigned any of the values that
 are listed in the declaration.  The CommandLine library enforces that
 the user can only specify one of the options, and it ensure that only valid enum
 values can be specified.  The "``clEnumVal``" macros ensure that the command
-line arguments matched the enum values.  With this option added, our help output
-now is:
+line arguments matched the enum values, and always remember to add "``clEnumValEnd``" 
+at the end since argument list for "``cl::values``" should be end with null.  
+With this option added, our help output now is:
 
 ::
 
   USAGE: compiler [options] <input file>
 
   OPTIONS:
     Choose optimization level:
       -g          - No optimizations, enable debugging
enum OptLevel {
     Debug, O1, O2, O3
   };
 
   cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
     cl::values(
      clEnumValN(Debug, "g", "No optimizations, enable debugging"),
       clEnumVal(O1        , "Enable trivial optimizations"),
       clEnumVal(O2        , "Enable default optimizations"),
-      clEnumVal(O3        , "Enable expensive optimizations")));
+      clEnumVal(O3        , "Enable expensive optimizations"), clEnumValEnd));
 
   ...
     if (OptimizationLevel == Debug) outputDebugInfo(...);
   ...
 
 By using the "``clEnumValN``" macro instead of "``clEnumVal``", we can directly
 specify the name that the flag should get.  In general a direct mapping is nice,
 but sometimes you can't or don't want to preserve the mapping, which is when you
enum DebugLev {
     nodebuginfo, quick, detailed
   };
 
   // Enable Debug Options to be specified on the command line
   cl::opt<DebugLev> DebugLevel("debug_level", cl::desc("Set the debugging level:"),
     cl::values(
       clEnumValN(nodebuginfo, "none", "disable debug information"),
        clEnumVal(quick,               "enable quick debug information"),
-       clEnumVal(detailed,            "enable detailed debug information")));
+       clEnumVal(detailed,            "enable detailed debug information"),
+       clEnumValEnd));
 
 This definition defines an enumerated command line variable of type "``enum
 DebugLev``", which works exactly the same way as before.  The difference here is
 just the interface exposed to the user of your program and the help output by
 the "``-help``" option:
 
 ::
 

 
 .. code-block:: c++
 
   cl::list<Opts> OptimizationList(cl::desc("Available Optimizations:"),
     cl::values(
       clEnumVal(dce               , "Dead Code Elimination"),
       clEnumVal(constprop         , "Constant Propagation"),
      clEnumValN(inlining, "inline", "Procedure Integration"),
-      clEnumVal(strip             , "Strip Symbols")));
+      clEnumVal(strip             , "Strip Symbols"), clEnumValEnd));
 
 This defines a variable that is conceptually of the type
 "``std::vector<enum Opts>``".  Thus, you can access it with standard vector
 methods:
 
 .. code-block:: c++
 
   for (unsigned i = 0; i != OptimizationList.size(); ++i)

 
 .. code-block:: c++
 
   cl::bits<Opts> OptimizationBits(cl::desc("Available Optimizations:"),
     cl::values(
       clEnumVal(dce               , "Dead Code Elimination"),
       clEnumVal(constprop         , "Constant Propagation"),
      clEnumValN(inlining, "inline", "Procedure Integration"),
-      clEnumVal(strip             , "Strip Symbols")));
+      clEnumVal(strip             , "Strip Symbols"), clEnumValEnd));
 
 To test to see if ``constprop`` was specified, we can use the ``cl:bits::isSet``
 function:
 
 .. code-block:: c++
 
   if (OptimizationBits.isSet(constprop)) {
     ...