[io] Fix `_NativeSocket.shutdownRead`

When shutting down receive direction we need to ensure that
`closedRead` event is dispatched - as the receive direction
is only considered closed if the available data is drained
and `closedRead` is dispatched.

The code did not account for a possibility that socket has
no available data and as such is not dispatching read events
so closing receive direction and then separately closing
send direction would leave the socket in a state where
both directions are closed but the socket is not disposed
because `closedRead` is not dispatched - such socket objects
will simply leak (even if the other side terminates
the connection).

This CL also updates documentation around `RawSocket.shutdown`
and `RawSocket.readEventsEnabled` to make it clear that
users are responsible for draining accumulated data if
they want to shutdown receive direction.

Fixes https://github.com/dart-lang/sdk/issues/27414

TEST=standalone/io/issue_27414

CoreLibraryReviewExempt: Documentation only changes in VM specific library
Change-Id: I4b0ffb4cc67836c2849ec6e49b788a4f3b4c07d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396340
Reviewed-by: Brian Quinlan <bquinlan@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>