Public alerts API

Update the following methods:

• alerts_namespace:add(alert) - now returns true.
• alerts_namespace:set(key, alert) - now returns true if the alert
  was set, false if an alert with the same key and message already
  exists.
• alerts_namespace:unset(key) - now returns true if the alert was
  discarded, false if it didn't exist.
• alerts_namespace:clear() - now returns true if any alerts were
  discarded, false if the namespace was already empty.

New method:

• alerts_namespace:get(key) - returns the alert with the given key,
  or nil if it doesn't exist.
  Requested by @Godrik0 in https://github.com/tarantool/tarantool/commit/5879f121b2c6babaefefed81d721b83e904e3308.

Find the existing documentation entry for the alerts_namespace API and compare its documented methods with the requested return values. Add the get(key) method and document the true/false outcomes for add, set, unset, and clear. Done means the public API reference matches all behaviors listed in the issue.