Ignore clippy::unnecessary_safety_doc lint

```
warning: safe function's docs have unnecessary `# Safety` section
   --> futures-task/src/future_obj.rs:151:5
    |
151 |     fn into_raw(self) -> *mut (dyn Future<Output = T> + 'a);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_safety_doc
    = note: `#[warn(clippy::unnecessary_safety_doc)]` on by default

warning: safe function's docs have unnecessary `# Safety` section
   --> futures-util/src/future/future/shared.rs:141:5
    |
141 |     pub fn strong_count(&self) -> Option<usize> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_safety_doc
    = note: `#[warn(clippy::unnecessary_safety_doc)]` on by default

warning: safe function's docs have unnecessary `# Safety` section
   --> futures-util/src/future/future/shared.rs:154:5
    |
154 |     pub fn weak_count(&self) -> Option<usize> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_safety_doc
```

Author: taiki-e

futures-task/src/future_obj.rs

@@ -148,6 +148,7 @@ pub unsafe trait UnsafeFutureObj<'a, T>: 'a {
     /// provided `*mut (dyn Future<Output = T> + 'a)` into a `Pin<&mut (dyn
     /// Future<Output = T> + 'a)>` and call methods on it, non-reentrantly,
     /// until `UnsafeFutureObj::drop` is called with it.
+    #[allow(clippy::unnecessary_safety_doc)]
     fn into_raw(self) -> *mut (dyn Future<Output = T> + 'a);
 
     /// Drops the future represented by the given fat pointer.

futures-util/src/future/future/shared.rs

@@ -138,6 +138,7 @@ where
     /// This method by itself is safe, but using it correctly requires extra care. Another thread
     /// can change the strong count at any time, including potentially between calling this method
     /// and acting on the result.
+    #[allow(clippy::unnecessary_safety_doc)]
     pub fn strong_count(&self) -> Option<usize> {
         self.inner.as_ref().map(Arc::strong_count)
     }
@@ -151,6 +152,7 @@ where
     /// This method by itself is safe, but using it correctly requires extra care. Another thread
     /// can change the weak count at any time, including potentially between calling this method
     /// and acting on the result.
+    #[allow(clippy::unnecessary_safety_doc)]
     pub fn weak_count(&self) -> Option<usize> {
         self.inner.as_ref().map(Arc::weak_count)
     }