Rollup merge of #38636 - shahn:extend, r=steveklabnik
Rollup merge of #38636 - shahn:extend, r=steveklabnik

Clarify Extend behaviour wrt existing keys

This seems to be consistent with all the Extend implementations I found, and isn't documented anywhere else afaik.

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