Document WrappingMode::NoWrapping's bool payload with a doc comment

The explanation of what the payload means used //, so rustdoc dropped it
from the public WrappingMode page. WrappingMode is re-exported from
lib.rs, so the variant is documented like the neighbouring enums.

src/wrapping.rs

@@ -2,7 +2,7 @@
 pub enum WrappingMode {
     Character,
     Word,
-    // The bool specifies whether wrapping has been explicitly disabled by the user via --wrap=never
+    /// The bool specifies whether wrapping has been explicitly disabled by the user via --wrap=never
     NoWrapping(bool),
 }