Rollup merge of #69405 - NieDzejkob:docs-readline-appends, r=joshtriplett
Rollup merge of #69405 - NieDzejkob:docs-readline-appends, r=joshtriplett

docs: Stdin::read_line: mention the appending

The fact that `stdin().read_line()` is an [unpleasant](https://twitter.com/Michcioperz/status/1231646797661167617?s=20) [footgun](https://rustbattle.net/battle/straight-finch-8-e4f4). Let's make it clearer in the documentation.

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