`parse_docs` doesn't handle block doc comments properly

[gustavo-shigueo]

The way we generate the doc string is:

format!("/**\n{}\n */\n", lines.join("\n")),

lines in the snippet represents a Vec<String>, where each item is the content of a single #[doc = "..."] attribute.

This works great with /// comments, as the compiler converts each of these to its own #[doc = "..."] attribute, where ... is replace with the contents of the comment.

However, a /***/ comment is replaced with a single #[doc = "..."] attribute, where ... is replaced with every single character between the second and third * characters, including any leading or trailing \n.

So

/**
 * Scan progress
 */

is replaced with #[doc = "\n * Scan progress\n "]. This leading \n, combined with our default \n + "* " before the #[doc] contents, causes the weird blank line

Originally posted by @gustavo-shigueo in #316 (comment)