Where are the rules about suspension? Maybe they should be in FAQ?
Not exactly; the most common reasons are documented here:
When users exhibit a pattern of either …
No effort to learn and improve over time
- This user does not put reasonable effort into the questions they ask of the community.
- There is little or no evidence of this user learning over time, either in the topic itself or in the community norms on the site.
- This user intentionally spams the site with the same question or very similar questions, over and over.
- The user never gives anything back to the community, but only takes.
- Other users tend to react poorly to this user’s contributions, posting negative responses in kind and generally causing a commotion.
- There is a broad sense of community resentment over this user’s behavior, and they are frequently cited in discussion about the community.
- There is a dark storm cloud of moderator flags that seems to follow this user around wherever they go.
- The moderators get email complaints about this user’s behavior.
- This user makes overtly snide, rude, or hostile comments to their fellow users.
… these problem behaviors have to be dealt with. When they aren’t, it takes up excessive moderator time that could be used for something more productive — and, even worse, these behaviors begin to actively turn people away from our community, stunting its growth and harming everyone.
Assuming you're asking about the rules governing when a user can be suspended: there is only one, "don't be a jerk." This is intentionally vague. Suspensions are imposed at the discretion of the moderators. Generally they only occur after an official warning has been given but has not had the desired effect.