Incorrect documentation for pull_type::operator= in Boost.Coroutine2

The documentation for pull_type::operator= in Boost.Coroutine2 is currently misleading. It states:
```
Effects: Destroys the internal data of *this and moves the internal data of other to *this. other becomes not-a-coroutine.
```

Real code:

```cpp
pull_coroutine & operator=( pull_coroutine && other) noexcept {
std::swap( cb_, other.cb_);
return * this;
}
```

Additionally, the absence of a .swap() member function in pull_type might lead to incorrect assumptions—particularly that
```cpp

using std::swap;
swap(a, b);
```
would invalidate both coroutines, which is not the case.

Doc:

https://www.boost.org/doc/libs/1_78_0/libs/coroutine2/doc/html/coroutine2/coroutine/asymmetric/pull_coro.html