Documentation/0.5: add api.md

- Update all of the ports to use the new IANA port numbers
- Update the stats section to talk about the id fields
- Don't mention /version until #1391 is fixed
- Remove mention of the modules
- Remove -L from all of the curl commands since it is no longer needed
- Point people at the clustering.md guide for the cluster APIs
Maybe we could mention they are IANA ports.