Rollup merge of #32136 - nathankleyn:improve-docs-for-btreeset, r=steveklabnik
Rollup merge of #32136 - nathankleyn:improve-docs-for-btreeset, r=steveklabnik

Add missing documentation examples for BTreeSet.

As part of the ongoing effort to document all methods with examples,
this commit adds the missing examples for the `BTreeSet` collection
type.

This is part of issue #29348.

r? @steveklabnik

GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust