Several slack receiver parameters need some explanation

[mpalmer]

The Slack incoming webhook documentation linked to in the alertmanager configuration doesn't mention several of the parameters available in the slack_configs section:

• color
• title
• title_link
• pretext
• fallback

While title and title_link I can guess at (first line of text, emphasised, and associated link), the color parameter's defaults are non-obvious ("danger" and "good"...), and pretext and fallback I can only guess at.

[robcomcast]

I've been struggling to figure them out myself.