Export of internal Abseil changes

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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

diff --git a/absl/flags/internal/usage.h b/absl/flags/internal/usage.h
index 0c62dc4..619ccce 100644
--- a/absl/flags/internal/usage.h
+++ b/absl/flags/internal/usage.h
@@ -36,7 +36,8 @@
   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,
               HelpFormat format = HelpFormat::kHumanReadable);