net: tcp: Add comment of func prototype for NET_CONN_CB macro usages

NET_CONN_CB() functional macro hides the parameters a function takes
and its return type. In #8723, it's proposed to remove that macro
altogether. Until that proposal is reviewed, at least provide real
protype in code comments to help people who read/analyze the code.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>