doc/nvmf_tgt_pg: Update the description about assigning the created qpair to a poll group

upon creation on the thread from which they were created. Most importantly, *a

poll group may only be accessed from the thread on which it was created.*

When spdk_nvmf_tgt_accept() detects a new connection, it will construct a new

struct spdk_nvmf_qpair object and call the user provided `new_qpair_fn`

callback for each new qpair. In response to this callback, the user must

assign the qpair to a poll group by calling spdk_nvmf_poll_group_add().

Remember, a poll group may only be accessed from the thread on which it was created,

so making a call to spdk_nvmf_poll_group_add() may require passing a message

to the appropriate thread.

When spdk_nvmf_tgt_accept() detects a new connection, it chooses an optimal

poll group by first calling spdk_nvmf_get_optimal_poll_group(), which calls down

into the transport, and then assigns the qpair to the optimal poll group by

calling spdk_nvmf_poll_group_add(). This all happens within the NVMe-oF target

library and the NVMe-oF target application is not required to do anything other

than continue to periodically poll spdk_nvmf_tgt_accept().

## Access Control