Rollup merge of #34919 - GuillaumeGomez:btree_map_doc, r=steveklabnik
Rollup merge of #34919 - GuillaumeGomez:btree_map_doc, r=steveklabnik

Add doc for btree_map types

Part of #29348.

r? @steveklabnik

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