Make rustdoc JSON Span column 1-based, just like line numbers
Fixes https://github.com/rust-lang/rust/issues/139906.
This PR does two things:
1. It makes column 1-indexed as well, just like lines.
2. It updates documentation about them to mention that they are 1-indexed.
I think it's better for coherency to have them both 1-indexed instead of the weird mix we used to have. Docs for `line` and `col` fields can be found [here](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Loc.html#structfield.line).
And finally: it adds a regression test to ensure they are indeed 1-indexed.
r? `@aDotInTheVoid`
d49361a798