Fix doc comments on callbacks

[thelink2012]

Once rust-lang/rust#23812 is fixed this should be reverted.

One can easily use the following regexp replace to convert between the two doc comments modes:

From comments to attributes:

([\t ]*)\/\/\/[ ]?(.*)
$1#[doc="$2"]

From attributes back to comments:

([\t ]*)#\[doc="(.*)"\]
$1/// $2