[2pt] rST reference: admonitions

[NickVolynkin]

Describe using admonitions in the docs

Reviewer: @

DoD

• give syntax and output example for each type that we use
• explain when to use it
• if a type is used very rarely, consider replacing it with a more common one
• if a type doesn't have CSS styles, update tarantool/tarantool.io#556