cmov: mark isize/usize impls of Cmov(Eq) as always available (#1387)

Via rustc itself:

    expected values for `target_pointer_width` are: `16`, `32`, and `64`

This adds a `doc(cfg(true))` attribute to the impls for `isize` and
`usize` to hide the `cfg` gating so it doesn't show up in the rustdoc,
but it's still retained to the crate compiles if rustc suddenly decides
to add support for e.g. `8` or `128`-bit wide pointers.

Author: tarcieri

cmov/src/lib.rs

@@ -201,14 +201,17 @@ macro_rules! impl_cmov_traits_for_size_type {
             target_pointer_width = "32",
             target_pointer_width = "64"
         ))]
+        #[cfg_attr(docsrs, doc(cfg(true)))]
         impl Cmov for $size {
             #[cfg(target_pointer_width = "16")]
+            #[allow(clippy::cast_possible_truncation)]
             #[inline]
             fn cmovnz(&mut self, other: &Self, condition: Condition) {
                 (*self as $int16).cmovnz(&(*other as $int16), condition);
             }
 
             #[cfg(target_pointer_width = "32")]
+            #[allow(clippy::cast_possible_truncation)]
             #[inline]
             fn cmovnz(&mut self, other: &Self, condition: Condition) {
                 (*self as $int32).cmovnz(&(*other as $int32), condition);
@@ -227,14 +230,17 @@ macro_rules! impl_cmov_traits_for_size_type {
             target_pointer_width = "32",
             target_pointer_width = "64"
         ))]
+        #[cfg_attr(docsrs, doc(cfg(true)))]
         impl CmovEq for $size {
             #[cfg(target_pointer_width = "16")]
+            #[allow(clippy::cast_possible_truncation)]
             #[inline]
             fn cmovne(&self, rhs: &Self, input: Condition, output: &mut Condition) {
                 (*self as $int16).cmovne(&(*rhs as $int16), input, output);
             }
 
             #[cfg(target_pointer_width = "32")]
+            #[allow(clippy::cast_possible_truncation)]
             #[inline]
             fn cmovne(&self, rhs: &Self, input: Condition, output: &mut Condition) {
                 (*self as $int32).cmovne(&(*rhs as $int32), input, output);