refactor: update SLRU cache documentation and improve capacity handling

Revised the documentation for `SlruCore` to clarify the removal policy and enhance method descriptions, including the addition of a `peek` method for non-promoting access to cache entries. Updated the capacity handling by renaming `protected_cap` to `capacity` for consistency and clarity. This change aims to improve understanding and usability of the SLRU cache policy while aligning with Rust API guidelines.

Author: TKorr