Resolve empty_line_after_doc_comments clippy lint in lexical

    warning: empty line after doc comment
       --> tests/../src/lexical/math.rs:277:5
        |
    277 | /     /// ADDITION
    278 | |
        | |_
    ...
    284 |       pub fn iadd_impl(x: &mut Vec<Limb>, y: Limb, xstart: usize) {
        |       ----------------------------------------------------------- the comment documents this function
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_doc_comments
        = note: `-W clippy::empty-line-after-doc-comments` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::empty_line_after_doc_comments)]`
        = help: if the empty line is unintentional remove it
    help: if the documentation should include the empty line include it in the comment
        |
    278 |     ///
        |

src/lexical/math.rs

@@ -272,9 +272,7 @@ mod scalar {
 mod small {
     use super::*;
 
-    // MULTIPLICATIION
-
-    /// ADDITION
+    // ADDITION
 
     /// Implied AddAssign implementation for adding a small integer to bigint.
     ///