Updated documentation for {replMode} parameter in {Runtime.evaluate}

R=yangguo@chromium.org

Change-Id: Icafeeccdcbe854d6986d3930ec6fcb2c856d274a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2072743
Auto-Submit: Simon Zünd <szuend@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66444}

include/js_protocol.pdl

@@ -1315,7 +1315,9 @@ domain Runtime
       experimental optional TimeDelta timeout
       # Disable breakpoints during execution.
       experimental optional boolean disableBreaks
-      # Reserved flag for future REPL mode support. Setting this flag has currently no effect.
+      # Setting this flag to true enables `let` re-declaration and top-level `await`.
+      # Note that `let` variables can only be re-declared if they originate from
+      # `replMode` themselves.
       experimental optional boolean replMode
     returns
       # Evaluation result.