Rollup merge of #91482 - JosephTLyons:update-HashMap-and-BTreeMap-documentation,...

Rollup merge of #91482 - JosephTLyons:update-HashMap-and-BTreeMap-documentation, r=yaahc

Update documentation to use `from()` to initialize `HashMap`s and `BTreeMap`s

As of Rust 1.56, `HashMap` and `BTreeMap` both have associated `from()` functions.  I think using these in the documentation cleans things up a bit.  It allows us to remove some of the `mut`s and avoids the Initialize-Then-Modify anti-pattern.