gstrfuncs: Improve and port g_set_str() docs to gi-docgen
Fixes link and code block and includes code syntax.
Improves documentation to be more in line with the developer guidelines.
Since this function is currently not availabe in the docs (see #3059), this is hard to preview. But still, I figured while doing !3807 (merged), I might as well improve these docs in a separate MR. Here is a rendered preview:
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