Export of internal Abseil changes

--
9b45aa0bd58cd9e8478abffafa792e1e53feed08 by Abseil Team <absl-team@google.com>:

Document that command line flag help for a flag will include the default value.
Cleaned up the doc for undefok.

PiperOrigin-RevId: 335701801
GitOrigin-RevId: 9b45aa0bd58cd9e8478abffafa792e1e53feed08
Change-Id: I435e240bd10bf6a232a95f220cab83c41701ecb6

absl/flags/internal/usage.h

@@ -36,7 +36,8 @@ enum class HelpFormat {
   kHumanReadable,
   kHumanReadable,
 };
 };
 
 
-// Outputs the help message describing specific flag.
+// Streams the help message describing `flag` to `out`.
+// The default value for `flag` is included in the output.
 void FlagHelp(std::ostream& out, const CommandLineFlag& flag,
 void FlagHelp(std::ostream& out, const CommandLineFlag& flag,
               HelpFormat format = HelpFormat::kHumanReadable);
               HelpFormat format = HelpFormat::kHumanReadable);