callbacks: Improve the documentation of ssh_threads_set_callbacks().
callbacks: Improve the documentation of ssh_threads_set_callbacks().

BUG: https://red.libssh.org/issues/123

pouete/libssh,wangshawn/libssh,Distrotech/libssh,robxu9/libssh,Distrotech/libssh,DouglasHeriot/libssh,kedazo/libssh,mwgoldsmith/ssh,mwgoldsmith/libssh,DouglasHeriot/libssh,kedazo/libssh,mwgoldsmith/libssh,mwgoldsmith/ssh,DouglasHeriot/libssh,bigcat26/libssh-mod,wangshawn/libssh,kedazo/libssh,robxu9/libssh,mwgoldsmith/ssh,sebadoom/libssh,Distrotech/libssh,mwgoldsmith/ssh,pouete/libssh,nviennot/libssh,taikoo/libssh,taikoo/libssh,bigcat26/libssh-mod,nviennot/libssh,DouglasHeriot/libssh,elastichosts/libssh,wangshawn/libssh,wangshawn/libssh,nviennot/libssh,robxu9/libssh,elastichosts/libssh,bigcat26/libssh-mod,mwgoldsmith/libssh,pouete/libssh,taikoo/libssh,nviennot/libssh,Distrotech/libssh,taikoo/libssh,sebadoom/libssh,elastichosts/libssh,sebadoom/libssh,bigcat26/libssh-mod,sebadoom/libssh,kedazo/libssh,mwgoldsmith/libssh,robxu9/libssh,pouete/libssh