Rust-clippy: Reword help message for len_zero

[gyscos]

When calling .len() == 0 on something with a .is_empty() method, a lint is triggered with the following help message:

help: using `is_empty` is more concise: `self.buffer.is_empty()`

Mentioning concision rather than clarity is a bit misleading; .len() == 0 and .is_empty() have actually the exact same number of characters.

I suggest rewording it like this:

help: using `is_empty` is clearer and more explicit: `self.buffer.is_empty()`

(Run with clippy 0.0.212 (32b1d1f 2018-10-05))

[mrbuzz]

Hi I'd like to take this as my first issue.

[CYBAI]

As #3326 merged, maybe this can be closed? 😄