Clarify docs for 'zero_token'.

Fixes #1132.


[SVN r67774]

doc/overview.xml

@@ -181,7 +181,7 @@ options_description desc;
 desc.add_options()
     ("help", "produce help message")
     ("compression", value<string>(), "compression level")
-    ("verbose", value<string>()->zero_tokens(), "verbosity level")
+    ("verbose", value<string>()->implicit_value("0"), "verbosity level")
     ("email", value<string>()->multitoken(), "email to send to")
     ;
       </programlisting>

include/boost/program_options/value_semantic.hpp

@@ -261,13 +261,21 @@ namespace boost { namespace program_options {
             return this;
         }
 
-        /** Specifies that the value can span multiple tokens. */
+        /** Specifies that the value can span multiple tokens. 
+        */
         typed_value* multitoken()
         {
             m_multitoken = true;
             return this;
         }
 
+        /** Specifies that no tokens may be provided as the value of
+            this option, which means that only presense of the option
+            is significant. For such option to be useful, either the
+            'validate' function should be specialized, or the 
+            'implicit_value' method should be also used. In most
+            cases, you can use the 'bool_switch' function instead of
+            using this method. */
         typed_value* zero_tokens() 
         {
             m_zero_tokens = true;

src/cmdline.cpp

@@ -277,7 +277,7 @@ namespace boost { namespace program_options { namespace detail {
         }
 
         /* If an key option is followed by a positional option,
-           can can consume more tokens (e.g. it's multitoke option),
+           can can consume more tokens (e.g. it's multitoken option),
            give those tokens to it.  */
         vector<option> result2;
         for (unsigned i = 0; i < result.size(); ++i)