[Alerting][Docs] Updates to improve clarity of Alerting API documentation

[ymao1]

Some of our Alerting API documentation can be improved to be clearer:

• To create a rule through the API, you must specify a rule_type_id, but it is unclear what the possible set of rule_type_ids are. Could update the docs to explicitly list all the possibilities for rule_type_id or add a section to suggest using the List rule types API to determine which ones the user is able to create.
• Improve documentation around throttle value:
  • Clarify what types of strings the throttle value accepts.
  • Clarify what the various options in throttle do

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)