Improve docs around File and BufReader

[steveklabnik]

Part of #29356 and related to #29300

Inspired by https://www.reddit.com/r/rust/comments/4yinbt/writing_a_jpeg_decoder_in_rust_part_2/d6obhd8?st=is4wut7k&sh=6d31802c

BufReader's talks about why you'd need it, but it's not super clear from File, which is more likely how people will discover this stuff. We should make sure people using File as a Read understand things here.

[marti1125]

Hi @steveklabnik I would like contribute with this =D

[steveklabnik]

@marti1125 awesome! Please let me know if you need any help

[marti1125]

Hi @steveklabnik could you give some step how to improve this documentation please

[steveklabnik]

Hey @marti1125:

So, the main issue here is explaining in File's docs why you might want to pair the File with BufReader. I would approach this in two steps:

1. Mention in File's docs that you might want to combine it with BufReader, and link to Bufreader's docs
2. update BufReader's docs to be a little more clear about what's going on. It's very terse at the moment.

Does that make sense?

[frewsxcv]

Opened a PR that addresses this: #38443.