Rollup merge of #42159 - Havvy:doc-drop, r=steveklabnik
Rollup merge of #42159 - Havvy:doc-drop, r=steveklabnik

Document drop more.

Adds two examples to Drop and describes the recursive drop on types that contain fields.

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