'iff' for docs to 'if and only if'

src/libcore/num/mod.rs

@@ -1126,7 +1126,7 @@ macro_rules! uint_impl {
             acc
         }
 
-        /// Returns `true` if `self == 2^k` for some `k`.
+        /// Returns `true` if and only if `self == 2^k` for some `k`.
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
         pub fn is_power_of_two(self) -> bool {

src/libcore/str/pattern.rs

@@ -739,7 +739,7 @@ struct TwoWaySearcher {
     (We also allow for the possibility of the *empty word*, a word of length zero).
 
     If x is any non-empty word, then an integer p with 0 < p <= |x| is said to be a
-    *period* for x if for all i with 0 <= i <= |x| - p - 1, we have x[i] == x[i+p].
+    *period* for x iff for all i with 0 <= i <= |x| - p - 1, we have x[i] == x[i+p].
     For example, both 1 and 2 are periods for the string "aa". As another example,
     the only period of the string "abcd" is 4.