Update alert_action.html.markdown

[SpawW]

Add information about unicity of channel_id for telegram

[SpawW]

Sample error when we try to use the same channel ID in 2 different alert actions
╷
2│ Error: could not create an alert action with ID , error: Bad request: api respond with status code: 400, error code: ERROR, message: This 'channelId' is already set in another Telegram alert action, remove it first
3│
4│ with ilert_alert_action.aa_dev_telegram,
5│ on aa_dev_telegram.tf line 1, in resource "ilert_alert_action" "aa_dev_telegram":
6│ 1: resource "ilert_alert_action" "aa_dev_telegram" {
7│

[STLVRTX]

Hello, sorry for the late response, and thank you for the PR!

We're currently evaluating if we lift this restriction in our Telegram integration. When a decision was made I will provide an update comment on this PR. For now we can leave this open until a decision is made and then adjacently adjust the docs.