Fix false positive in empty_line_after_outer_attr

[phansch]

See #2340 (comment)

Doc comments are syntactic sugar for #[doc] attributes, so this lint was
catching them, too.

This commit makes it so that doc comments are ignored in this lint.

I think, for normal attributes it makes sense to warn about following empty
lines, for doc comments, less. This way the user has some freedom over
the formatting.

[phansch]

Another way to fix this could be to change the lint message if it is a doc comment (Found an empty line after a documentation comment ...). I'm not really sure what's better, though.

[oli-obk]

I'm not so sure about this one. I think doc comments should also be directly attached to their items without newlines inbetween. But I'm willing to go with it.

Alternatively we could make it two lints, one for doc comments and one for attributes and make the one for doc comments allow-by-default

[kpp]

I got tons of warnings. https://github.com/tox-rs/tox/blob/master/src/toxencryptsave/mod.rs#L75

warning: Found an empty line after an outer attribute. Perhaps you forgot to add a '!' to make it an inner attribute?
  --> src/toxencryptsave/mod.rs:75:1
   |
75 | / /** Bytes used to verify whether given data has been encrypted using **TES**.
76 | |     
77 | |     Located at the beginning of the encrypted data.
78 | | */
79 | | pub const MAGIC_NUMBER: &[u8; MAGIC_LENGTH] = b"toxEsave";
   | |_
   |
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.184/index.html#empty_line_after_outer_attr