Clarify documentation on std::io::read_line

[d33tah]

Take a look at this SO answer and the comments:

http://stackoverflow.com/a/41210857/1091116

[steveklabnik]

tagging p-high, @frewsxcv is gonna take this one. Not a big thing, but we'll knock it out soon.

[frewsxcv]

This seems to be already addressed in #36851. I also opened #40642 that should clarify the behaviors of these BufRead methods. So I think this can be considered done unless anyone else has an action item here.