Rollup merge of #96129 - mattheww:2022-04_float_rounding, r=Dylan-DPC

Rollup merge of #96129 - mattheww:2022-04_float_rounding, r=Dylan-DPC

Document rounding for floating-point primitive operations and string parsing

The docs for floating point don't have much to say at present about either the precision of their results or rounding behaviour.

As I understand it[^1][^2], Rust doesn't support operating with non-default rounding directions, so we need only describe roundTiesToEven.

[^1]: https://github.com/rust-lang/rust/issues/41753#issuecomment-299322887
[^2]: https://github.com/llvm/llvm-project/issues/8472#issuecomment-980888781

This PR makes a start by documenting that for primitive operations and `from_str()`.