Resolve "Document values of `MergeRequest.mergeStatus`"

What does this MR do?

This MR improves the end-user documentation for MergeRequest.mergeStatus, in both REST and GraphQL.

For REST, the user documentation is updated.

For GraphQL, and new enum type is introduced, which makes it clear what the values are, and how they should be interpreted.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #328815 (closed)