Document panicking cases for integer remainder

The panic when the right operand is `0` is expected, but the
overflow-related panic (which occurs only for `MIN % -1`) is somewhat
surprising for two reasons: a return value of `0` would be reasonable
in this case, and, for most arithmetic operations, overflow results in
panic only when `debug_assertions` is enabled. As a result, it's
helpful to document this behavior.

Author: jturner314

core/src/ops/arith.rs

@@ -556,9 +556,13 @@ pub trait Rem<Rhs = Self> {
 }
 
 macro_rules! rem_impl_integer {
-    ($($t:ty)*) => ($(
+    ($(($($t:ty)*) => $panic:expr),*) => ($($(
         /// This operation satisfies `n % d == n - (n / d) * d`. The
         /// result has the same sign as the left operand.
+        ///
+        /// # Panics
+        ///
+        #[doc = $panic]
         #[stable(feature = "rust1", since = "1.0.0")]
         impl Rem for $t {
             type Output = $t;
@@ -568,10 +572,13 @@ macro_rules! rem_impl_integer {
         }
 
         forward_ref_binop! { impl Rem, rem for $t, $t }
-    )*)
+    )*)*)
 }
 
-rem_impl_integer! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
+rem_impl_integer! {
+    (usize u8 u16 u32 u64 u128) => "This operation will panic if `other == 0`.",
+    (isize i8 i16 i32 i64 i128) => "This operation will panic if `other == 0` or if `self / other` results in overflow."
+}
 
 macro_rules! rem_impl_float {
     ($($t:ty)*) => ($(