MergeRequest API documentation incorrectly lists state_event options

Description

The MergeRequest API documentation lists the options as (close|reopen|merge), but merge has been moved to a separate endpoint.

Additionally they are not mentioned at all in doc/api/merge_requests.md since https://gitlab.com/gitlab-org/gitlab-ce/commit/8b5c16e4b1d54745ba6ca65ecfbf14c1683db3b4

Links / references

• https://gitlab.com/gitlab-org/gitlab-ce/commit/ab032256da9f24351871143058343f7463f9d7fc
• https://gitlab.zendesk.com/agent/tickets/52552
• https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/740/diffs