Document time of back operations of a Linked List
Document time of back operations of a Linked List

Popping and pushing from the end of a linked list is constant time. This
documentation is already there for popping and pushing from the front.

@bors: r+ 38fe8d2 rollup

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