Clarify `{Ord,f32,f64}::clamp` docs a little

Explicitly call out when it returns NaN, adhere to the panic doc
guidelines.

src/libcore/cmp.rs

@@ -568,8 +568,14 @@ fn min(self, other: Self) -> Self
         if self <= other { self } else { other }
     }
 
-    /// Returns max if self is greater than max, and min if self is less than min.
-    /// Otherwise this will return self.  Panics if min > max.
+    /// Restrict a value to a certain interval.
+    ///
+    /// Returns `max` if `self` is greater than `max`, and `min` if `self` is
+    /// less than `min`. Otherwise this returns `self`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `min > max`.
     ///
     /// # Examples
     ///
@@ -586,8 +592,7 @@ fn clamp(self, min: Self, max: Self) -> Self
         assert!(min <= max);
         if self < min {
             min
-        }
-        else if self > max {
+        } else if self > max {
             max
         } else {
             self

src/libstd/f32.rs

@@ -960,17 +960,27 @@ pub fn acosh(self) -> f32 {
     pub fn atanh(self) -> f32 {
         0.5 * ((2.0 * self) / (1.0 - self)).ln_1p()
     }
-    /// Returns max if self is greater than max, and min if self is less than min.
-    /// Otherwise this returns self.  Panics if min > max, min equals NaN, or max equals NaN.
+
+    /// Restrict a value to a certain interval unless it is NaN.
+    ///
+    /// Returns `max` if `self` is greater than `max`, and `min` if `self` is
+    /// less than `min`. Otherwise this returns `self`.
+    ///
+    /// Not that this function returns NaN if the initial value was NaN as
+    /// well.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `min > max`, `min` is NaN, or `max` is NaN.
     ///
     /// # Examples
     ///
     /// ```
     /// #![feature(clamp)]
-    /// assert!((-3.0f32).clamp(-2.0f32, 1.0f32) == -2.0f32);
-    /// assert!((0.0f32).clamp(-2.0f32, 1.0f32) == 0.0f32);
-    /// assert!((2.0f32).clamp(-2.0f32, 1.0f32) == 1.0f32);
-    /// assert!((std::f32::NAN).clamp(-2.0f32, 1.0f32).is_nan());
+    /// assert!((-3.0f32).clamp(-2.0, 1.0) == -2.0);
+    /// assert!((0.0f32).clamp(-2.0, 1.0) == 0.0);
+    /// assert!((2.0f32).clamp(-2.0, 1.0) == 1.0);
+    /// assert!((std::f32::NAN).clamp(-2.0, 1.0).is_nan());
     /// ```
     #[unstable(feature = "clamp", issue = "44095")]
     #[inline]

src/libstd/f64.rs

@@ -882,17 +882,26 @@ pub fn atanh(self) -> f64 {
         0.5 * ((2.0 * self) / (1.0 - self)).ln_1p()
     }
 
-    /// Returns max if self is greater than max, and min if self is less than min.
-    /// Otherwise this returns self.  Panics if min > max, min equals NaN, or max equals NaN.
+    /// Restrict a value to a certain interval unless it is NaN.
+    ///
+    /// Returns `max` if `self` is greater than `max`, and `min` if `self` is
+    /// less than `min`. Otherwise this returns `self`.
+    ///
+    /// Not that this function returns NaN if the initial value was NaN as
+    /// well.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `min > max`, `min` is NaN, or `max` is NaN.
     ///
     /// # Examples
     ///
     /// ```
     /// #![feature(clamp)]
-    /// assert!((-3.0f64).clamp(-2.0f64, 1.0f64) == -2.0f64);
-    /// assert!((0.0f64).clamp(-2.0f64, 1.0f64) == 0.0f64);
-    /// assert!((2.0f64).clamp(-2.0f64, 1.0f64) == 1.0f64);
-    /// assert!((std::f64::NAN).clamp(-2.0f64, 1.0f64).is_nan());
+    /// assert!((-3.0f64).clamp(-2.0, 1.0) == -2.0);
+    /// assert!((0.0f64).clamp(-2.0, 1.0) == 0.0);
+    /// assert!((2.0f64).clamp(-2.0, 1.0) == 1.0);
+    /// assert!((std::f64::NAN).clamp(-2.0, 1.0).is_nan());
     /// ```
     #[unstable(feature = "clamp", issue = "44095")]
     #[inline]