Document errors

[tobiasvl]

After #98, Result is returned in a lot of new places. These functions should probably document when errors can occur, and why.

According to clippy's missing_errors_doc lint, "Documenting the type of errors that can be returned from a function can help callers write code to handle the errors appropriately." It suggests doing this with a /// # Errors section in the rustdoc comments. I suggest following this suggestion.

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Background: I just upgraded to syntect 5.0.0, and I got a warning that HighlightLines::highlight had been "renamed" to HighlightLines::highlight_line. However, when I changed the function name, I noticed that the new function returned a Result, unlike the old function, and it wasn't readily apparent why. I checked the documentation to see if there were any new failure states, but didn't find any documentation. Only by checking the source code did I learn that the old function would panic, but I still don't know what could cause that to happen.

(Note that missing_panics_doc is a similar lint for panics, which also would have helped in this case, although I understand syntect has eliminated most/all panics by now.)