Fix -early-data documentation.

Change-Id: I76a87ebf2f8be731d6da2381710c1caa60298f6e
Reviewed-on: https://boringssl-review.googlesource.com/22924
Reviewed-by: Steven Valdez <svaldez@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

tool/client.cc

diff --git a/tool/client.cc b/tool/client.cc
index 873cea0..ea4d7fe 100644
--- a/tool/client.cc
+++ b/tool/client.cc
@@ -121,7 +121,8 @@
         "verification is required.",
     },
     {
-        "-early-data", kOptionalArgument, "Allow early data",
+        "-early-data", kOptionalArgument, "Enable early data. The argument to "
+        "this flag is the early data to send.",
     },
     {
         "-tls13-variant", kOptionalArgument,