simplify std::io::Write::write rustdoc

The std::io::Write::write method currensly suggests consumers guaranteed
that `0 <= n <= buf.len()`, for `Ok(n)`, however `n` is of type `usize`
causing the compiler to emit a warning:
```
warning: comparison is useless due to type limits
 --> lib.rs:6:18
  |
6 |         Ok(n) => 0 <= n && n <= output.len(),
  |                  ^^^^^^
  |
  = note: #[warn(unused_comparisons)] on by default
```

This PR removes the suggestion to check `0 <= n` since it is moot.

r? @steveklabnik

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