Rollup merge of #121634 - RavuAlHemio:slice-prefix-suffix-docs, r=cuviper

Clarify behavior of slice prefix/suffix operations in case of equality

Operations such as starts_with, ends_with, strip_prefix and strip_suffix can be either strict (do not consider a slice to be a prefix/suffix of itself) or not. In Rust's case, they are not strict. Add a few phrases to the documentation to clarify this.

Author: matthiaskrgr