WL#2733: Multi-source: Documentation

SUMMARY
-------
Document all the changes in server behaviour caused by the new 
multi-source implementation.

CHANGELOG CHANGES
-----------------
Guilhem wrote in his review:
> >    if (opt_bin_log && !server_id)
> >    {
> > -    server_id= !master_host ? 1 : 2;
> > +    server_id= 1;
> 
> [G] We'll need a load of changelog entries: for example, upgrading from
> [G] 5.0 to 5.1 may change the server id (as the implicit values changed),
> [G] causing the user some problems.

MANUAL CHANGES
--------------
- The default named server always exists, so there will always 
  be at least one line of SHOW SLAVE STATUS output.
- RESET SLAVE has new semantics
- The variable Slave_running has different semantics: 
  when checking the value of this variable, 
  the MASTER variable controls which named master 
  is checked.
- slave_skip_errors applies to all named masters