fix missing_docs warning for cfg(all(loom, test)) functions

```
error: missing documentation for an associated function
   --> src/bytes.rs:146:5
    |
146 |     pub fn new() -> Self {
    |     ^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D missing-docs` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(missing_docs)]`

error: missing documentation for an associated function
   --> src/bytes.rs:176:5
    |
176 |     pub fn from_static(bytes: &'static [u8]) -> Self {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

src/bytes.rs

@@ -142,6 +142,7 @@ impl Bytes {
         Bytes::from_static(EMPTY)
     }
 
+    /// Creates a new empty `Bytes`.
     #[cfg(all(loom, test))]
     pub fn new() -> Self {
         const EMPTY: &[u8] = &[];
@@ -172,6 +173,7 @@ impl Bytes {
         }
     }
 
+    /// Creates a new `Bytes` from a static slice.
     #[cfg(all(loom, test))]
     pub fn from_static(bytes: &'static [u8]) -> Self {
         Bytes {