Use bold instead of quotes

In an effort to assist readability in rendered HTML docs use bold markup
instead of quotes. Also use italics around 'iff' for the same reason.

src/lib.rs

@@ -333,14 +333,14 @@ impl Signature {
         }
     }
 
-    /// Normalizes a signature to a "low S" form. In ECDSA, signatures are
+    /// Normalizes a signature to a __low s__ form. In ECDSA, signatures are
     /// of the form (r, s) where r and s are numbers lying in some finite
-    /// field. The verification equation will pass for (r, s) iff it passes
-    /// for (r, -s), so it is possible to ``modify'' signatures in transit
+    /// field. The verification equation will pass for (r, s) _iff_ it passes
+    /// for (r, -s), so it is possible to __modify__ signatures in transit
     /// by flipping the sign of s. This does not constitute a forgery since
     /// the signed message still cannot be changed, but for some applications,
     /// changing even the signature itself can be a problem. Such applications
-    /// require a "strong signature". It is believed that ECDSA is a strong
+    /// require a __strong signature__. It is believed that ECDSA is a strong
     /// signature except for this ambiguity in the sign of s, so to accommodate
     /// these applications libsecp256k1 will only accept signatures for which
     /// s is in the lower half of the field range. This eliminates the