Updated docs

  * tmux/tmux.org: added setting for 'command-prefix', it set to "",
    this new option without this value breaks my start-up scripts
    (nevertheless I need rethink all this stuff);
  * bash/profile.org: added check for 'TMUX' variable, if it is defined
    then skip the rest of start-up script code because it means that
    'tmux' arleady started (that code can be called from tmux when
    creating a window).

Signed-off-by: Vladimir Lomov <lomov.vl@gmail.com>