Markdown textarea footer does not mention that quick actions are supported even if they are

When creating comments we show:

When updating comments we show:

We have supported quick actions when editing comments for a while now so we should also mention that to make the feature discoverable.

Combined with #216707 (closed) where the autocomplete dropdown isn't working, there's no way for the user to know that quick actions are actually supported when editing comments.

!31186 (merged) adds support for quick actions when editing Issue, MR, and Epic descriptions. There's a similar problem noted in !31186 (comment 358663760) for issues and epics. We'd have to pass the quick actions doc link to the Vue component for it to show up.