Conflicting docs on performance of boxed

[peter-lyons-kehl]

Thank you for smartstring Bodil.

• https://docs.rs/smartstring/latest/smartstring/struct.Compact.html > This performs generally as well as String on all ops on boxed strings... (bold is mine), conflicts with:
• https://docs.rs/smartstring/latest/smartstring > There will always be a slight overhead on all operations on **boxed strings**, compared to [String](https://doc.rust-lang.org/nightly/alloc/string/struct.String.html) (again, bold is mine).