io: add doc warning about concurrently calling poll_read/write_ready (#2439)
io: add doc warning about concurrently calling poll_read/write_ready (#2439)

Co-authored-by: Alice Ryhl <522b276a356bdf39013dfabea2cd43e141ecc9e8@ryhl.io>
Fixes: #2429
tokio-rs/tokio,tokio-rs/tokio