Ignore clippy::doc_overindented_list_items lint

```
error: doc list item overindented
   --> pin-project-internal/src/pin_project/derive.rs:856:5
    |
856 | ///            instead of generating `Drop` impl.
    |     ^^^^^^^^^^^ help: try using `  ` (2 spaces)
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_overindented_list_items
    = note: `-D clippy::doc-overindented-list-items` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::doc_overindented_list_items)]`
```

.github/.cspell/project-dictionary.txt

@@ -1,6 +1,7 @@
 compiletest
 expandtest
 HRTB
+overindented
 projectable
 qself
 reborrow

pin-project-internal/src/pin_project/derive.rs

@@ -848,6 +848,7 @@ fn make_unpin_impl(cx: &Context<'_>) -> TokenStream {
     }
 }
 
+#[allow(clippy::doc_overindented_list_items)]
 /// Creates `Drop` implementation for the original type.
 ///
 /// The kind of `Drop` impl generated depends on `pinned_drop` field: