Clarify paragraph about sending REDACT in on-JOIN chathistory batch (#…

…539)

extensions/message-redaction.md

@@ -130,7 +130,7 @@ Such use cases might include:
 * Allowing channel moderators or server admins to delete unwelcome messages from others
 * Specifying a cut-off time after which message edits are no longer allowed
 
-If a message is redacted while a client is not present in a channel, servers may send the `REDACT` command in a `chathistory` batch when it re-joins the channel.
+Servers may send `REDACT` commands in [`chathistory` batches] sent to clients on joining a channel.
 
 If servers use predictable or guessable `msgid`s, they should consider whether errors
 returned on `REDACT` may leak a message's existence to users who did not receive it
@@ -204,3 +204,4 @@ Deleting someone else's PRIVMSG:
 [`message-tags`]: ../extensions/message-tags.html
 [`msgid`]: ../extensions/message-ids.html
 [`chathistory`]: ../extensions/chathistory.html
+[`chathistory` batches]: ../batches/chathistory.html