re-add -reindex help message

was suddenly removed with fc44231cb7 (probably not by purpose)

src/init.cpp

@@ -279,7 +279,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-prune=<n>", strprintf(_("Reduce storage requirements by pruning (deleting) old blocks. This mode disables wallet support and is incompatible with -txindex. "
             "Warning: Reverting this setting requires re-downloading the entire blockchain. "
             "(default: 0 = disable pruning blocks, >%u = target size in MiB to use for block files)"), MIN_DISK_SPACE_FOR_BLOCK_FILES / 1024 / 1024));
-
+    strUsage += HelpMessageOpt("-reindex", _("Rebuild block chain index from current blk000??.dat files on startup"));
 #if !defined(WIN32)
     strUsage += HelpMessageOpt("-sysperms", _("Create new files with system default permissions, instead of umask 077 (only effective with disabled wallet functionality)"));
 #endif