Rollup merge of #71892 - integer32llc:btreemap-entry-vacant-docs, r=jonas-schievink
Rollup merge of #71892 - integer32llc:btreemap-entry-vacant-docs, r=jonas-schievink

Update btree_map::VacantEntry::insert docs to actually call insert

It looks like they were copied from the `or_insert` docs. This change
makes the example more like the hash_map::VacantEntry::insert docs.

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