rust: account for doc comment syntax

[tomjakubowski]

Rust has two flavors of comment, the plain line comment (//) and the doc comment (///). Rustdoc, the native documentation tool, renders only doc comments (///).

docco now searches for both // comments and /// comments in Rust source files. Previously, it had only searched for // comments, so /// comments were rendered with an ugly superfluous / at the start of their content.