doc: Really minor updates to NVMe-oF programmers guide

## The Basics

A user of the NVMe-oF target library begins by creating a target using

spdk_nvmf_tgt_create(), setting up a set of addresses to accept connections on

by calling spdk_nvmf_tgt_listen(), then creating a subsystem using

spdk_nvmf_subsystem_create().

spdk_nvmf_tgt_create(), setting up a set of addresses on which to accept

connections by calling spdk_nvmf_tgt_listen(), then creating a subsystem

using spdk_nvmf_subsystem_create().

Subsystems begin in an inactive state and must be activated by calling

spdk_nvmf_subsystem_start(). Subsystems may be modified at run time, but only

network I/O. Poll groups may be created by calling

spdk_nvmf_poll_group_create(). They automatically request to begin polling

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

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

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 it was created on,

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.

spdk_nvmf_tgt_accept() can be handed out round-robin to the poll groups. This

is how the SPDK NVMe-oF target application currently functions.

More advanced algorithms for distributing qpairs to poll groups is possible.

More advanced algorithms for distributing qpairs to poll groups are possible.

For instance, a NUMA-aware algorithm would be an improvement over basic

round-robin, where NUMA-aware means assigning qpairs to poll groups running on

CPU cores that are on the same NUMA node as the network adapter and storage

## Zero Copy Support

For the RDMA transport, data is transferred from the RDMA NIC to host memory

and then host memory to the SSD (or vis. versa), without any intermediate

and then host memory to the SSD (or vice versa), without any intermediate

copies. Data is never moved from one location in host memory to another. Other

transports in the future may require data copies.