docs: clarify display precision contract

[tisonkun]

Summary

• distinguish exact ByteSize formatting from the approximate, f64-backed Display wrapper in the crate overview
• document precision loss, automatic scale selection, formatter precision, round-trip limits, and infinity behavior in the API docs
• verify that the unit-boundary bug fixed by fix: ideal_unit_std picks the wrong unit near power-of-unit boundaries bytesize-rs/bytesize#175 does not reproduce because bsize selects scales iteratively; no formatting behavior or tests change