integrations: Change Slack private channel mention string.

In Slack webhook integration setup, the only scenario where a message
payload contains a channel mention without the channels name is when the
user mentions a private channel.
  e.g <#C07AVLQ3AUQ|>

This commit updates the string we use for such mention to "private Slack
channel" for better clarity.

zerver/webhooks/slack/view.py

@@ -79,7 +79,9 @@ def convert_slack_user_and_channel_mentions(text: str, app_token: str) -> str:
             # a mention isn't triggered for a Zulip channel with the same name.
             channel_info: list[str] = slack_channelmention_match.group(0).split("|")
             channel_name = channel_info[1]
-            tokens[iterator] = f"**#{channel_name}**" if channel_name else "**#[private channel]**"
+            tokens[iterator] = (
+                f"**#{channel_name}**" if channel_name else "**#[private Slack channel]**"
+            )
     text = " ".join(tokens)
     return text