Rollup merge of #137126 - m4rch3n1ng:fix-inherent-str-docs, r=Amanieu

fix docs for inherent str constructors

related to #131114

when implementing inherent str constructors in #136517, i forgot to change the docs, so the code examples still imported the `std::str` module and used the constructor from there, instead of using "itself" (the inherent constructor).

Author: matthiaskrgr