MR Discussions API: document parameters "line_code" & "line_type"

What does this MR do?

The "line_range" parameter needlessly exposes implementation details (#247521 (closed)), and should probably be reworked. I started working on that, but I don't know when I can finish it, hence this patch to improve the documentation of the existing API.

I'd argue that most users are already happy when they can create notes on single lines. Me and others (#219215 (closed)) missed the fact that the "line_range" is optional. Make that very explicit.

The first commit cleans up bad API usage in a test I added in !47130 (merged).

Related issues

#247521 (closed), #219215 (closed)

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