Do not parse a line with only `/`s as a doc comment

[lifthrasiir]

A line filled with only /s is occasionally used as a separator in the code. Rust currently parses it as a doc comment which causes unwanted headaches, as shown below:

////////////////////////////////////////
// common macros

macro_rules! foo( ... ) // error: macro_rules! do not support #[doc = ...]
macro_rules! bar( ... )

While such separators should be avoided in favor of the module system, it is still useful for logically splitting large functions for instance. Also related to #4106.