Fix BufRead::read_line unconventional behaviour

Hello,

I've read the documentation so I understand that read_line appends into the passed &mut String argument. I don't understand the use case of that behaviour. To circumvent it, one has to write its own little helper function or not forget to .clear() or .truncate(0) before each new call. So read_line's name doesn't match is behaviour because it reads and appends.

I also looked at the tests 1 and 2 for read_line and they don't even verify that append feature because they .truncate(0) between each check.

Would it be possible to deprecate that behaviour ? If not, would it be possible to document the design intent with a sensible use case, add tests for it and think about an additional simpler interface to load a line into a String ?

Thank you.

Start with the referenced read_line tests in src/libstd/io/mod.rs and src/libstd/io/buffered.rs, and compare them with the documented append behavior described in the issue. Done requires a decided, scoped direction among deprecation, improved documentation and tests, or an additional interface; the current issue does not identify one implementation path.